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>

capabilities/gemini/capability.json

@@ -1,75 +0,0 @@
-{
-  "id": "gemini",
-  "role": "runtime",
-  "version": "1.7.0-rc.2",
-  "title": "Gemini CLI",
-  "description": "Google Gemini CLI — commands-only artifact layout (TOML); Gemini hook event dialect; settings-json hook surface; tier-2 support.",
-  "tier": "core",
-  "requires": [],
-  "engines": {
-    "gsd": ">=1.6.0"
-  },
-  "runtime": {
-    "configHome": {
-      "kind": "dot-home",
-      "name": ".gemini",
-      "env": [
-        "GEMINI_CONFIG_DIR"
-      ]
-    },
-    "localConfigDir": ".gemini",
-    "configFormat": "settings-json",
-    "artifactLayout": {
-      "global": [
-        {
-          "kind": "commands",
-          "destSubpath": "commands/gsd",
-          "prefix": "gsd-",
-          "nesting": "flat",
-          "recursive": false,
-          "converter": null
-        }
-      ],
-      "local": [
-        {
-          "kind": "commands",
-          "destSubpath": "commands/gsd",
-          "prefix": "gsd-",
-          "nesting": "flat",
-          "recursive": false,
-          "converter": null
-        }
-      ]
-    },
-    "commandStyle": "slash-hyphen",
-    "hooksSurface": "settings-json",
-    "hookEvents": "gemini",
-    "sandboxTier": "none",
-    "supportTier": 2,
-    "installSurface": "settings-json",
-    "writesSharedSettings": true,
-    "permissionWriter": null,
-    "extendedHookEvents": [
-      "BeforeAgent",
-      "AfterAgent",
-      "BeforeModel"
-    ],
-    "hostIntegration": {
-      "embeddingMode": "declarative",
-      "commandSurface": "slash-toml",
-      "dispatch": {
-        "namedDispatch": true,
-        "nested": false,
-        "maxDepth": 1,
-        "background": "undocumented",
-        "subagentToolkit": "undocumented",
-        "backgroundDispatch": false
-      },
-      "modelMode": "passive",
-      "hookBus": "host",
-      "stateIO": "filesystem",
-      "transport": "mcp",
-      "runtime": "node"
-    }
-  }
-}