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>

gsd-core/bin/shared/model-catalog.json

@@ -17,11 +17,6 @@
       "sonnet": { "model": "gpt-5.4", "reasoning_effort": "medium" },
       "haiku": { "model": "gpt-5.4-mini", "reasoning_effort": "medium" }
     },
-    "gemini": {
-      "opus": { "model": "gemini-3.1-pro-preview" },
-      "sonnet": { "model": "gemini-3-flash" },
-      "haiku": { "model": "gemini-2.5-flash-lite" }
-    },
     "qwen": {
       "opus": { "model": "qwen3-max-2026-01-23" },
       "sonnet": { "model": "qwen3-coder-plus" },

gsd-core/bin/shared/runtime-aliases.manifest.json

@@ -13,11 +13,6 @@
     "kilo",
     "kilo-cli"
   ],
-  "gemini": [
-    "gemini",
-    "gemini-cli",
-    "gemini-code"
-  ],
   "codex": [
     "codex",
     "codex-app",