feat(#1928): remove sunset Gemini CLI runtime, redirect to Antigravity (#1996)

* feat(#1928): remove sunset gemini cli runtime, redirect to antigravity

Google sunset Gemini CLI on 2026-06-18; Antigravity CLI is its official successor (already a first-class GSD runtime). Remove the gemini runtime from the enum (16->15), aliases, labels, config-home fragment, install path, converters (convertClaudeToGemini{Markdown,Toml,Agent}, convertSlashCommandsToGeminiMentions), capability descriptor, gemini-extension.json, RULESET.GEMINI.*, and the interactive menu (renumbered, no gap).

--gemini now prints an explicit deprecation notice citing the 2026-06-18 sunset and redirects to --antigravity (no silent alias, per the issue's Hyrum's-Law rejection). Antigravity is preserved throughout: its GEMINI.md contextFileName, .gemini/antigravity config home, the shared convertGeminiToolName/claudeToGeminiTools tool vocabulary, and the 'gemini' hookEvents dialect it declares. GEMINI.md retargeted as Antigravity's context file.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* chore(#1928): backfill changeset PR number (#1996)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* chore(#1928): drop Gemini CLI from issue templates (review nit)

Removes the sunset Gemini CLI runtime from the two GitHub issue-template
runtime lists that the removal PR missed, per @davesienkowski's review nit:
- feature_request.yml: 'Applicable runtimes' checkbox (a user could otherwise
  request a feature for a runtime GSD no longer supports)
- bug_report.yml: 'Runtime' dropdown + the stale ~/.gemini/settings.json
  retrieval-help line

Leaves the post-removal templates fully consistent with the Antigravity redirect.

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

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/runtime-artifact-layout-descriptor-drive.test.cjs

@@ -3,7 +3,7 @@
 /**
  * Equivalence proof for ADR-857 phase 5d: descriptor-driven resolveRuntimeArtifactLayout.
  *
- * For every runtime in the 16-entry capability registry × {global, local} scopes,
+ * For every runtime in the 15-entry capability registry × {global, local} scopes,
  * this test asserts that:
  *   1. kind.kind, kind.destSubpath, kind.prefix are byte-identical to the STEP-0
  *      golden captured from the old switch() before any edits.
@@ -11,11 +11,11 @@
  *   3. layout.runtime === runtime, layout.configDir === configDir, layout.scope === scope.
  *
  * SCOPE-FALL-THROUGH NOTE:
- *   The old switch() had no scope branches for 13 runtimes (cursor, gemini, codex,
+ *   The old switch() had no scope branches for 12 runtimes (cursor, codex,
  *   copilot, antigravity, windsurf, augment, trae, qwen, hermes, codebuddy, opencode,
  *   kilo), meaning scope='local' returned the same kinds as scope='global'. The 5a
  *   descriptors incorrectly set local:[] for those runtimes, causing 31 local-install
- *   test regressions. The 5b backfill sets local == global for these 13, restoring
+ *   test regressions. The 5b backfill sets local == global for these 12, restoring
  *   the old switch's scope-agnostic behaviour.
  *
  *   For runtimes that had explicit scope branches in the old switch
@@ -72,15 +72,6 @@ const GOLDEN = {
     { kind: 'agents',   destSubpath: 'agents',   prefix: 'gsd-' },
   ],
 
-  // ── gemini ───────────────────────────────────────────────────────────────────
-  // Old switch: no scope branch → local == global. 5b backfill restores this.
-  'gemini/global': [
-    { kind: 'commands', destSubpath: 'commands/gsd', prefix: 'gsd-' },
-  ],
-  'gemini/local': [
-    { kind: 'commands', destSubpath: 'commands/gsd', prefix: 'gsd-' },
-  ],
-
   // ── codex ────────────────────────────────────────────────────────────────────
   // Old switch: no scope branch → local == global. 5b backfill restores this.
   'codex/global': [
@@ -217,7 +208,7 @@ const GOLDEN = {
 // ── Parametrized tests ────────────────────────────────────────────────────────
 
 const RUNTIMES = [
-  'claude', 'cursor', 'gemini', 'codex', 'copilot',
+  'claude', 'cursor', 'codex', 'copilot',
   'antigravity', 'windsurf', 'augment', 'trae', 'qwen',
   'hermes', 'codebuddy', 'cline', 'kimi', 'opencode', 'kilo',
 ];