fix(#3738): antigravity global skills/agents install to ~/.gemini/config (#3921)

* test(#3738): antigravity global skills/agents must resolve under ~/.gemini/config

Regression tests (RED first): --skills-root and gsd-tools query surfaces,
install-plan dest dirs, and converter skills-path rewrite.

* fix(#3738): antigravity global skills/agents install to ~/.gemini/config

Antigravity's machine-local discovery scans ~/.gemini/config/{skills,agents};
the configHome (~/.gemini/antigravity) is deprecated for artifacts. Declare the
ADR-1239 skills/agents 'home' override on the antigravity global layout — the
same mechanism codex uses (.agents) — and divert ~/.claude/skills/ references
in converted global content to ~/.gemini/config/skills/. configHome, settings,
probe/migration semantics, and the local .agents layout are unchanged.

* fix(#3738): retire deprecated configHome artifacts via installer migration 010

Next install converges an existing antigravity install: manifest-managed
skills/gsd-*/ and agents/gsd-*.md under the configHome (a location AGY does
not scan) are removed — modified files backed up first, unmanifested and
non-gsd entries preserved — and now-empty containers retired. Global scope
only; the local .agents surface is live. Docs + inventory updated.

* fix(#3738): converter sync in bin/install.js, harness emit-root coverage, migration baseline

- bin/install.js converter gains the same ~/.claude/skills → ~/.gemini/config/
  rewrite as src (ADR-1508 dual copy must stay in sync).
- Parity-manifest walk covers home-override emit roots (extraEmitRootsFor) so
  antigravity's emitted skills/agents stay differential-visible at their new
  install root; install-tree fixture regen confirms an unchanged key set.
- skills-from-commands rule declares the antigravity converter as a
  runtime-scoped transform; one ack fragment covers the identity-classed
  workflow whose antigravity copy embeds the old skills path.
- Migration 010 checksum baseline + home-override set doc updated; existing
  tests updated to the #3738 contract (global dest, golden parity via layout
  dest, integration expectations).

* fix(#3738): tolerate an absent extra emit root on baseline-side measurement

The base tree's installer predates the home override, so <HOME>/.gemini/config
does not exist there; walk() threw ENOENT and the in-job baseline build failed.
An absent extra root is the legitimate pre-override shape — skip it.

* fix(#3738): review findings — manifest agents root, bare skills-path rewrite, guard comment

- writeManifest resolves the agents-kind home override (_kindDestDirSafe), so
  the manifest records agents at their actual install root and drift detection
  keeps working (isolated review finding 1, major).
- Converter bare forms ~/.claude/skills and $HOME/.claude/skills (no trailing
  slash) divert to ~/.gemini/config/skills instead of falling through to the
  retired configHome path (finding 2).
- real-home-guard comment updated: antigravity's global agents kind is the
  first agents-kind home override (finding 3, doc-only).
- Regression tests for both behavioral findings.

* chore(#3738): changeset fragment (pr number backfilled after PR creation)

* chore(#3738): backfill changeset PR number (3921)

* fix(#3738): sandbox HOME in tests that install antigravity global artifacts

antigravity is the first home-override runtime in the golden-parity and
skills-wrapper suites (codex is not in their runtime lists), so those tests
never needed HOME sandboxing — the real-home guard now (correctly) refuses
their un-sandboxed global installs on CI, where HOME is the passwd home.

* fix(#3738): stop the K3 sequential-sandbox env leak; sandbox L2's home-override plans

K3's two back-to-back sandboxHome calls leave HOME pointing at the first
sandbox once the after-hooks restore (each call saves the env as it found
it, so the second saves the first's sandbox as 'original'). On the windows
matrix that leaked gsd-k3-qwen-* home into the L2 property, whose
antigravity/global run then (correctly) refused via the #3712 real-home
guard — antigravity is the runtime that made L2's plan escape into
os.homedir(). K3 now manages the env with a single restore; L2 sandboxes
HOME per run, mirroring L1.

* fix(#3738): L2 property's HOME sandbox must exist on disk

The #3712 guard's sandbox exemption fails closed when identify(effectiveHome)
is 'absent' — L2 never created its configDir, so on the windows matrix (tmpdir
under the real home) the antigravity/global run refused even with HOME
sandboxed. Create the per-run sandbox dir and clean it up.

---------

Co-authored-by: sim <sim@local>

tests/executed-plan.test.cjs

@@ -1165,12 +1165,29 @@ describe('installRuntimeArtifacts — K3: real install before/after, full recurs
     for (const runtime of ['claude', 'qwen']) {
       const dirA = createTempDir(`gsd-k3-${runtime}-a-`);
       const dirB = createTempDir(`gsd-k3-${runtime}-b-`);
-      t.after(() => { cleanup(dirA); cleanup(dirB); });
+      // Two SEQUENTIAL sandboxes for one test: sandboxHome()'s per-call
+      // t.after hooks each save the env as they found it, so the second call
+      // saves the FIRST sandbox as its "original" — hook ordering then leaves
+      // HOME pointing at dirA after the test, leaking into later tests in the
+      // shard (observed on the windows matrix: a leaked gsd-k3-qwen-* home
+      // made the L2 property's antigravity/global run refuse via the
+      // #3712 real-home guard). Manage the env directly with ONE restore.
+      const savedHome = process.env.HOME;
+      const savedUserProfile = process.env.USERPROFILE;
+      const savedMarker = process.env.GSD_TEST_HOME_SANDBOX;
+      t.after(() => {
+        if (savedHome === undefined) delete process.env.HOME; else process.env.HOME = savedHome;
+        if (savedUserProfile === undefined) delete process.env.USERPROFILE; else process.env.USERPROFILE = savedUserProfile;
+        if (savedMarker === undefined) delete process.env.GSD_TEST_HOME_SANDBOX; else process.env.GSD_TEST_HOME_SANDBOX = savedMarker;
+        cleanup(dirA); cleanup(dirB);
+      });
 
-      sandboxHome(t, dirA);
-      installRuntimeArtifacts(runtime, dirA, 'global', RESOLVED_FULL);
-      sandboxHome(t, dirB);
-      installRuntimeArtifacts(runtime, dirB, 'global', RESOLVED_FULL);
+      for (const dir of [dirA, dirB]) {
+        process.env.HOME = dir;
+        process.env.USERPROFILE = dir;
+        process.env.GSD_TEST_HOME_SANDBOX = dir;
+        installRuntimeArtifacts(runtime, dir, 'global', RESOLVED_FULL);
+      }
 
       const filesA = walkFilesRecursively(dirA);
       const filesB = walkFilesRecursively(dirB);
@@ -1260,17 +1277,36 @@ describe('installRuntimeArtifacts — L2: plan is deterministic (property)', ()
     };
   }
 
-  test('plan is deterministic', () => {
+  test('plan is deterministic', (t) => {
     const runtimes = Object.keys(registry.runtimes);
     const RUNTIME_ARB = fc.constantFrom(...runtimes);
     const SCOPE_ARB = fc.constantFrom('global', 'local');
     let hits = 0;
+    // #3738: the per-run HOME sandbox must EXIST on disk — the #3712 guard's
+    // sandbox exemption fails closed when it cannot stat the effective home
+    // (identify() -> 'absent'), which is exactly what a never-created configDir
+    // gives it on the windows matrix where tmpdir sits under the real home.
+    const createdL2Dirs = [];
+    t.after(() => { for (const d of createdL2Dirs) cleanup(d); });
 
     fc.assert(
       fc.property(RUNTIME_ARB, SCOPE_ARB, (runtime, scope) => {
         // configDir is never created for real — both calls run against fresh,
         // independent fake adapters, so no real fs cleanup is needed here.
         const configDir = path.join(os.tmpdir(), `gsd-l2-${runtime}-${crypto.randomUUID()}`);
+        fs.mkdirSync(configDir, { recursive: true });
+        createdL2Dirs.push(configDir);
+        // #3738: a home-override runtime (antigravity → <HOME>/.gemini/config)
+        // resolves its dest from os.homedir(), NOT configDir — sandbox HOME to
+        // configDir for the duration of both calls (mirroring L1 above) so the
+        // plan never escapes into an ambient or leaked home and the #3712
+        // real-home guard stays satisfied on hosts where tmpdir sits under the
+        // real home (windows).
+        const savedL2Home = process.env.HOME;
+        const savedL2UserProfile = process.env.USERPROFILE;
+        process.env.HOME = configDir;
+        process.env.USERPROFILE = configDir;
+        try {
         const planA = installRuntimeArtifacts(runtime, configDir, scope, RESOLVED_CORE, undefined, undefined, { fs: createFakeInstallFs() });
         const planB = installRuntimeArtifacts(runtime, configDir, scope, RESOLVED_CORE, undefined, undefined, { fs: createFakeInstallFs() });
         hits++;
@@ -1281,6 +1317,10 @@ describe('installRuntimeArtifacts — L2: plan is deterministic (property)', ()
           `L2 (${runtime}/${scope}): two installs against fresh fake adapters with the same inputs must ` +
           'yield structurally identical plans (temp-dir names normalized — see normalizePlanForIdempotence)',
         );
+        } finally {
+          if (savedL2Home === undefined) delete process.env.HOME; else process.env.HOME = savedL2Home;
+          if (savedL2UserProfile === undefined) delete process.env.USERPROFILE; else process.env.USERPROFILE = savedL2UserProfile;
+        }
       }),
       { numRuns: 30, seed: 2874, verbose: true },
     );