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>

scripts/sync-manifest-versions.cjs

@@ -24,15 +24,16 @@ const ROOT = path.resolve(__dirname, '..');
 // Single source of truth: runtime-integration manifests whose `version` field
 // MUST track package.json. Each entry names the manifest path plus the dotted
 // path to the version field inside that JSON document. Top-level `version` is
-// the default (plugin.json, gemini-extension.json); the Claude plugin
+// the default (plugin.json); the Claude plugin
 // marketplace manifest carries its canonical version at plugins[0].version
 // (the schema-canonical location runtimes read — issue #1855).
 //
 // Add a new manifest here so `npm version` keeps it in sync — the regression
 // guard test (issue 844) fails if you forget.
+// #1928: gemini-extension.json was removed with the gemini runtime (Google
+// sunset Gemini CLI 2026-06-18); it is no longer a registered manifest.
 const VERSIONED_MANIFESTS = [
   { path: '.claude-plugin/plugin.json', versionKey: 'version' },
-  { path: 'gemini-extension.json', versionKey: 'version' },
   { path: '.claude-plugin/marketplace.json', versionKey: 'plugins.0.version' },
   { path: 'vscode/package.json', versionKey: 'version' },
 ];