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>

docs/explanation/context-engineering.md

@@ -77,7 +77,6 @@ Since GSD 1.4.0, this is addressed by registering runtime lifecycle hooks. Rathe
 Each AI runtime exposes lifecycle events in its own vocabulary, but the purpose is the same across all of them: fire at boundaries that correspond to context pressure or turn transitions, so GSD can observe and react.
 
 - **Claude Code** fires `PreCompact` when a compaction is about to occur, `Stop` when a session turn ends, and `SubagentStop` when a spawned subagent completes. Together these bracket the moments when context has grown or a context-consuming task has just finished.
-- **Gemini** fires `BeforeAgent`/`AfterAgent` around each agent invocation, and `BeforeModel` before each model call — giving a per-inference opportunity to check headroom.
 - **Qwen** exposes `SubagentStop`, `Stop`, and `PreCompact`, mirroring Claude Code's shape in its own event system.
 
 Think of these as the same concept — "notify GSD at context boundaries" — expressed in each runtime's native event vocabulary. This is the multi-runtime philosophy applied at the observability layer: GSD registers the semantically equivalent hook wherever each runtime exposes it, rather than demanding every runtime adopt a single event schema.