feat(#2096): migrate Antigravity onto EoS declarative adapter + permission-writer + MCP companion (ADR-1239)

Fold all antigravity literal branches into descriptor-driven reads:
getConfigDirFromHome (→ configHome.kind 'dot-home-nested'), projectLocalHookPrefix
(→ hostBehaviors.hookPathStyle 'raw'), applyAgentPathRewrites (→ noPathRewrite),
getProjectInstructionFile (→ projectInstructionFile 'GEMINI.md'); removed the dead
inline convertClaudeAgentToAntigravityAgent branch + dead isAntigravity
destructures (antigravity is already on the descriptor-agents path). subagentToolkit
flipped undocumented→full (Context7: antigravity.google/docs/cli/features);
namedDispatch/nested/maxDepth/backgroundDispatch stay undocumented. Byte-identical
golden parity for all 16 runtimes.

UPGRADE 1 (permission-writer): permissionWriter 'antigravity' + configureAntigravityPermissions
merges a scoped permissions.allow block (GSD's own tree + hooks) into Antigravity's
settings.json — non-destructive, idempotent, symmetric uninstall. Added to
VALID_PERMISSION_WRITERS + the FinishPermissionWriter union.
UPGRADE 2 (MCP companion): configureAntigravityMcpConfig writes mcp_config.json
registering the gsd-core companion MCP server (Gemini-successor mcpServers schema,
best-effort — raw schema unpublished). Both writers dispatch from finishInstall.
settings.json is golden-excluded (HOOK_CONFIG_FILES); mcp_config.json (portable,
no absolute paths) is golden-tracked → only antigravity.json changes.

Tests: declarative-reference-antigravity extended (source-grep guard across 4
modules, fail-closed for the 4 undocumented sub-axes, validator acceptance) +
antigravity-upgrades (permission-writer + mcp_config live-install, idempotency,
user-preservation). Matrix + ADR-1016 + capability-manifest + CONTEXT.md +
connect-gsd-mcp-server docs updated; changeset (Changed).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/runtime-artifact-conversion.cts

@@ -2743,9 +2743,11 @@ function normalizeAgentBodyForRuntime(content: string, runtime: string, cmdNames
  *   ~/\.claude\b → normalizedPathPrefix
  *   $HOME/\.claude\b → normalizedPathPrefix
  *
- * Skipped for copilot and antigravity (which do NOT do path rewrites in the
- * inline loop). NO stamp (_stampNonClaudeRuntimeDefaults) — agents are NOT
- * stamped in the inline loop.
+ * Skipped for copilot (hardcoded — #2099 will fold it) and for any runtime
+ * that declares `hostBehaviors.noPathRewrite` (descriptor-driven, ADR-1239 /
+ * #2096 — folds the prior hardcoded `runtime === 'antigravity'` literal;
+ * Antigravity does NOT do path rewrites in the inline loop). NO stamp
+ * (_stampNonClaudeRuntimeDefaults) — agents are NOT stamped in the inline loop.
  *
  * ADR-1235 §1: pre-converter cross-cutting for descriptor-driven agent pipeline.
  * Exported as `applyAgentPathRewrites` for testing and for injection into
@@ -2754,10 +2756,10 @@ function normalizeAgentBodyForRuntime(content: string, runtime: string, cmdNames
  * @param content     raw agent file content
  * @param runtime     canonical runtime ID
  * @param pathPrefix  trailing-slash path prefix (e.g. '$HOME/.cursor/')
- * @returns content with path-prefix rewrites applied (or unchanged for copilot/antigravity)
+ * @returns content with path-prefix rewrites applied (or unchanged for copilot / noPathRewrite runtimes)
  */
 function applyAgentPathRewrites(content: string, runtime: string, pathPrefix: string): string {
-  if (runtime === 'copilot' || runtime === 'antigravity') return content;
+  if (runtime === 'copilot' || _hostBehaviors(runtime).noPathRewrite === true) return content;
   const normalizedPathPrefix = pathPrefix.replace(/\/$/, '');
   content = content.replace(/~\/\.claude\//g, pathPrefix);
   content = content.replace(/\$HOME\/\.claude\//g, pathPrefix);

src/runtime-config-adapter-registry.cts

@@ -22,9 +22,12 @@
  *     false for codex / copilot / kilo / cursor / windsurf / trae / cline / kimi (legacy exclusion list).
  *     true for all other runtimes.
  * - `finishPermissionWriter` names the finishInstall-phase dedicated config writer:
- *     'opencode' → writes BOTH shared settings AND its own permissions file.
- *     'kilo'     → writes only its own permissions file.
- *     null       → no dedicated permission writer.
+ *     'opencode'    → writes BOTH shared settings AND its own permissions file.
+ *     'kilo'        → writes only its own permissions file.
+ *     'antigravity' → writes BOTH shared settings.json permissions.allow AND a
+ *                      standalone mcp_config.json MCP companion profile (#2096
+ *                      Phase B Upgrades 1+2).
+ *     null          → no dedicated permission writer.
  */
 
 // eslint-disable-next-line @typescript-eslint/no-require-imports
@@ -45,7 +48,7 @@ type ConfigInstallSurface =
   | 'cursor-hooks-json'
   | 'profile-marker-only';
 
-type FinishPermissionWriter = 'opencode' | 'kilo' | null;
+type FinishPermissionWriter = 'opencode' | 'kilo' | 'antigravity' | null;
 
 type HooksSurface =
   | 'settings-json'

src/runtime-name-policy.cts

@@ -120,13 +120,26 @@ export function resolveRuntimeNameFromCandidates(...candidates: unknown[]): stri
  * Aliases are normalized via `canonicalizeRuntimeName` first, so inputs like
  * `codex-cli` resolve to `codex` → `AGENTS.md`. Replaces the prior codex-only
  * override in profile-output.cjs (#3163) which left AGENTS-native runtimes
- * (opencode/kilo/kimi) incorrectly emitting `.claude/CLAUDE.md`. Pure: no I/O.
+ * (opencode/kilo/kimi) incorrectly emitting `.claude/CLAUDE.md`. Pure: no I/O
+ * (the lazy `require` below reads a static generated module, not the disk).
+ *
+ * Descriptor-driven (ADR-1239 / #2096): antigravity's `GEMINI.md` is folded
+ * from a hardcoded `canonical === 'antigravity'` literal into a read of
+ * `runtime.hostBehaviors.projectInstructionFile`. claude/copilot stay
+ * hardcoded (out of scope here) mirroring `getDirName` below, which already
+ * lazy-`require`s `capability-registry.cjs` inside the function body to
+ * avoid a circular dependency at module load.
  */
 export function getProjectInstructionFile(runtime: unknown): string {
   const canonical = canonicalizeRuntimeName(runtime);
   if (canonical === 'claude') return '.claude/CLAUDE.md';
   if (canonical === 'copilot') return '.github/copilot-instructions.md';
-  if (canonical === 'antigravity') return 'GEMINI.md';
+  // eslint-disable-next-line @typescript-eslint/no-require-imports
+  const { runtimes } = require('./capability-registry.cjs') as {
+    runtimes: Record<string, { runtime?: { hostBehaviors?: { projectInstructionFile?: string } } } | undefined>;
+  };
+  const declared = canonical ? runtimes[canonical]?.runtime?.hostBehaviors?.projectInstructionFile : undefined;
+  if (typeof declared === 'string' && declared.length > 0) return declared;
   // codex, opencode, kilo, kimi, AND unknown/future runtimes all default to
   // root AGENTS.md (the safe cross-agent instruction file).
   return 'AGENTS.md';

src/shell-command-projection.cts

@@ -98,9 +98,17 @@ export function formatManagedHookScriptToken(scriptPath: string, opts: { platfor
   return JSON.stringify(scriptPath.replace(/\\/g, '/'));
 }
 
-export function projectLocalHookPrefix({ runtime = 'claude', dirName }: { runtime?: string; dirName?: string | null }): string | undefined | null {
+export function projectLocalHookPrefix({ runtime: _runtime = 'claude', dirName, hookPathStyle }: { runtime?: string; dirName?: string | null; hookPathStyle?: string | null }): string | undefined | null {
   if (!dirName) return dirName;
-  return (runtime === 'antigravity')
+  // Descriptor-driven (ADR-1239 / #2096): folded from a hardcoded
+  // `runtime === 'antigravity'` literal into the runtime's declared
+  // `hostBehaviors.hookPathStyle`. Runtimes that always run project hooks
+  // with the project dir as cwd (Antigravity today) declare 'raw' and get
+  // the bare dirName; every other runtime keeps the $CLAUDE_PROJECT_DIR-
+  // anchored prefix. `runtime` itself is now unused here but stays in the
+  // signature for call-site/back-compat parity (kept `_`-prefixed to
+  // satisfy no-unused-vars).
+  return (hookPathStyle === 'raw')
     ? dirName
     : `"$CLAUDE_PROJECT_DIR"/${dirName}`;
 }