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>

src/shell-command-projection.cts

@@ -24,16 +24,20 @@ import childProcess from 'node:child_process';
  * runtime/shell combination.
  *
  * Current evidence-backed policy:
- * - Gemini on Windows requires `& ` for quoted node/bash runners.
- * - Claude Code on Windows does NOT: its hook commands execute under bash/Git
- *   Bash and `& ` breaks there (#3413).
+ * - Claude Code on Windows does NOT need it: its hook commands execute under
+ *   bash/Git Bash and `& ` breaks there (#3413).
+ * - #1928: Gemini CLI — the ONLY runtime with a verified need for the `& `
+ *   prefix on Windows — was removed (Google sunset it 2026-06-18). No currently
+ *   supported runtime has a verified need, so this seam is now inert. It is
+ *   retained (not deleted) so a future runtime with a verified need is a
+ *   one-line re-enable, per the conservative policy below. Note: Antigravity —
+ *   the Gemini-backend successor — never matched the old `runtime === 'gemini'`
+ *   check, so its behavior (no prefix) is unchanged.
  *
  * Keep the policy conservative until another runtime has a verified need.
  */
-export function hookCommandNeedsPowerShellCallOperator(opts: { platform?: string; runtime?: string } = {}): boolean {
-  const platform = opts.platform || process.platform;
-  const runtime = opts.runtime || 'generic';
-  return platform === 'win32' && runtime === 'gemini';
+export function hookCommandNeedsPowerShellCallOperator(_opts: { platform?: string; runtime?: string } = {}): boolean {
+  return false;
 }
 
 /**
@@ -96,7 +100,7 @@ export function formatManagedHookScriptToken(scriptPath: string, opts: { platfor
 
 export function projectLocalHookPrefix({ runtime = 'claude', dirName }: { runtime?: string; dirName?: string | null }): string | undefined | null {
   if (!dirName) return dirName;
-  return (runtime === 'gemini' || runtime === 'antigravity')
+  return (runtime === 'antigravity')
     ? dirName
     : `"$CLAUDE_PROJECT_DIR"/${dirName}`;
 }