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/golden-parity-single-source.test.cjs

@@ -132,7 +132,7 @@ const { installRuntimeArtifacts } = require('../gsd-core/bin/lib/install-engine.
 const { applySurface } = require('../gsd-core/bin/lib/surface.cjs');
 const { loadSkillsManifest, resolveProfile } = require('../gsd-core/bin/lib/install-profiles.cjs');
 const { resolveRuntimeArtifactLayout } = require('../gsd-core/bin/lib/runtime-artifact-layout.cjs');
-const { cleanup } = require('./helpers.cjs');
+const { cleanup, sandboxHome } = require('./helpers.cjs');
 
 const COMMANDS_GSD = path.join(ROOT, 'commands', 'gsd');
 
@@ -171,12 +171,22 @@ describe('#1575 — golden-parity: surface path matches install path for descrip
     test(`${runtime}: surface agents byte-identical to install agents`, (t) => {
       const configDir = fs.mkdtempSync(path.join(os.tmpdir(), `gsd-1575-${runtime}-`));
       t.after(() => { try { cleanup(configDir); } catch { /* best-effort */ } });
+      // #3738: antigravity's skills/agents kinds declare a global `home`
+      // override resolved from os.homedir() — sandbox HOME to the configDir
+      // (the #3712 marker real-home-guard needs) so the override resolves
+      // inside the sandbox instead of the runner's real home.
+      sandboxHome(t, configDir);
 
       // Step 1: install path writes agents
       installRuntimeArtifacts(runtime, configDir, 'global', parity1575Profile, resolveAttribution1575);
 
-      // Step 2: snapshot agent files
-      const agentsDir = path.join(configDir, 'agents');
+      // Step 2: snapshot agent files at the installer's REAL destination —
+      // honor the kind `home` override (codex → ~/.agents, antigravity →
+      // ~/.gemini/config per #3738) exactly like assertDestWithinConfigHome's
+      // root selection, never assume configDir/agents.
+      const parityLayout = resolveRuntimeArtifactLayout(runtime, configDir, 'global');
+      const parityAgentsKind = parityLayout.kinds.find((k) => k.kind === 'agents');
+      const agentsDir = path.join(parityAgentsKind.home ?? configDir, parityAgentsKind.destSubpath);
       const installSnap = snapshotAgents(agentsDir);
       assert.ok(installSnap.size > 0, `${runtime}: install must produce at least one gsd-* agent`);