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>

GEMINI.md

@@ -1,9 +1,13 @@
-# GSD Core — Gemini CLI context
+# GSD Core — Antigravity CLI context
 
-This context is loaded by the **gsd-core Gemini CLI extension**. It gives Gemini
-the operating context for [GSD Core](https://github.com/open-gsd/gsd-core), a
-meta-prompting, context-engineering, and spec-driven development system for AI
-coding agents.
+> **Gemini CLI was sunset by Google on 2026-06-18** and is no longer served for
+> free/Pro/Ultra tiers. Antigravity CLI is its official successor, and this file
+> is the context Antigravity reads automatically (its `contextFileName` is
+> `GEMINI.md`, inherited from the shared Gemini 3 backend).
+
+This context gives Antigravity the operating context for
+[GSD Core](https://github.com/open-gsd/gsd-core), a meta-prompting,
+context-engineering, and spec-driven development system for AI coding agents.
 
 ## What GSD is
 
@@ -16,30 +20,28 @@ files rather than in the conversation.
 
 ## The slash commands (installed separately)
 
-> **This extension ships only the context above — not the slash commands.** It
-> loads gsd's operating context into your Gemini sessions and is managed through
-> `gemini extensions list / update / uninstall`. To install the `/gsd:*` command
-> set, agents, and hooks into `~/.gemini/`, run the dedicated installer:
+> **This file ships only the context above — not the slash commands.** To
+> install the `/gsd-*` command set, agents, and hooks into `~/.gemini/antigravity/`,
+> run the dedicated installer:
 >
 > ```bash
-> npx gsd-core --gemini --global
+> npx gsd-core --antigravity --global
 > ```
 >
-> The two paths are complementary and the manual installer remains fully
-> supported. The commands below are available only once that installer has run.
+> The commands below are available only once that installer has run.
 
-If you have installed the gsd commands, the workflow is driven by these `/gsd:*`
-slash commands (Gemini registers gsd's commands under the `gsd` namespace, so the
-colon form is canonical):
+If you have installed the gsd commands, the workflow is driven by these `/gsd-*`
+slash commands (Antigravity registers gsd's commands under a hyphenated
+namespace):
 
-- `/gsd:new-project` — initialise a project and gather deep context.
-- `/gsd:progress` — the unified situational command: check progress, advance the
+- `/gsd-new-project` — initialise a project and gather deep context.
+- `/gsd-progress` — the unified situational command: check progress, advance the
   workflow, or dispatch a freeform intent.
-- `/gsd:plan-phase <N>` — produce a detailed phase plan with a verification loop.
-- `/gsd:execute-phase <N>` — execute a phase's plans with wave-based parallelism.
-- `/gsd:verify-work` — validate built features through conversational UAT.
-- `/gsd:ship` — open a PR, run review, and prepare for merge.
-- `/gsd:help` — list every available command.
+- `/gsd-plan-phase <N>` — produce a detailed phase plan with a verification loop.
+- `/gsd-execute-phase <N>` — execute a phase's plans with wave-based parallelism.
+- `/gsd-verify-work` — validate built features through conversational UAT.
+- `/gsd-ship` — open a PR, run review, and prepare for merge.
+- `/gsd-help` — list every available command.
 
 ## Working with GSD
 
@@ -47,7 +49,7 @@ colon form is canonical):
   acting, and keep it current as work progresses.
 - Prefer the smallest change that satisfies the phase's verification criteria.
 - Run the project's tests and linters before declaring a phase done.
-- When unsure what to do next, and the gsd commands are installed, `/gsd:progress`
+- When unsure what to do next, and the gsd commands are installed, `/gsd-progress`
   is the situational entry point.
 
 Learn more: <https://github.com/open-gsd/gsd-core>