feat(#791): migrate antigravity workspace base dir .agent → .agents (#1090)

Fresh antigravity workspace installs write under the canonical .agents/ (plural) base; legacy .agent/ stays recognized (dual-read). Global ~/.gemini/antigravity/ path unchanged. Closes #791.

docs/installer-migrations.md

@@ -371,7 +371,7 @@ for the new shape before changing migration behavior.
 | Kimi CLI | Agent Skills in `skills/gsd-*/SKILL.md`; explicit custom agent YAML/prompt artifacts in `agents/gsd.yaml`, `agents/gsd.md`, and `agents/subagents/gsd-*`; `gsd-core/` payload files referenced by generated skills; manifest, pristine, local-patch, and migration journal files from the normal installer safety pipeline | Global `KIMI_CONFIG_DIR`, explicit `--config-dir`, or first-existing generic skills root: `~/.config/agents` when `~/.config/agents/skills` exists or no generic skills root exists yet, otherwise `~/.agents` when `~/.agents/skills` exists and `~/.config/agents/skills` does not; `KIMI_CONFIG_DIR` and `--config-dir` are GSD write-location overrides and arbitrary roots require Kimi-side `--skills-dir` or `extra_skill_dirs` configuration for skill discovery; local `--kimi --local` is guarded and writes no project-level artifacts | GSD owns only generated `skills/gsd-*`, `agents/gsd.*`, `agents/subagents/gsd-*`, installed `gsd-core/` payload files, and manifest/preservation/migration records. GSD does not own Kimi config files, hooks, settings, rules, statusline, update-banner registration, or non-GSD Kimi skills/agents. Reinstall/update must preserve locally modified generated Kimi artifacts through manifest-backed `gsd-local-patches/`; uninstall removes only GSD-owned Kimi artifacts and preserves non-GSD user content. | [Agent Skills](https://moonshotai.github.io/kimi-cli/en/customization/skills.html), [Agents and Subagents](https://moonshotai.github.io/kimi-cli/en/customization/agents.html), [Tools](https://moonshotai.github.io/kimi-code/en/reference/tools.html); docs checked 2026-06-07 |
 | Codex | Skills in `skills/gsd-*/SKILL.md`; agents as source markdown plus per-agent TOML in `agents/`; `[agents.gsd-*]` and hooks in `config.toml` | Global `CODEX_HOME` or `~/.codex`; local `./.codex` | GSD owns generated skills, generated agent TOML, `agents.gsd-*` config sections, `[features].hooks` when added by GSD (canonical; legacy alias `codex_hooks` is recognized and migrated forward, #3566), and GSD hook entries | [Codex config schema](https://developers.openai.com/codex/config-schema.json), [Codex developer docs](https://developers.openai.com/codex/); docs not versioned, checked 2026-05-15; installer compatibility sentinel: Codex 0.130.0 features.hooks key (legacy `codex_hooks` recognized) |
 | GitHub Copilot | Skills in `skills/gsd-*/SKILL.md`; agents as `.agent.md`; repository instructions in `copilot-instructions.md` | Global `COPILOT_CONFIG_DIR`, `COPILOT_HOME`, or `~/.copilot`; local `./.github` | GSD owns generated skill/agent files and GSD-authored instruction files; no hook/statusline ownership | [Repository custom instructions](https://docs.github.com/en/copilot/how-tos/configure-custom-instructions/add-repository-instructions), [Copilot CLI custom instructions](https://docs.github.com/en/copilot/how-tos/copilot-cli/add-custom-instructions); GitHub Docs product docs, checked 2026-05-11 |
-| Antigravity | Skills in `skills/gsd-*/SKILL.md`; agents in `agents/`; Gemini-style `settings.json` hooks when installed by GSD | Global `ANTIGRAVITY_CONFIG_DIR` or `~/.gemini/antigravity`; local `./.agent` | GSD owns generated skills/agents/hooks and GSD settings entries only | Public Antigravity install/config docs for this file layout were not stable or complete as of 2026-05-11; installer compatibility therefore uses GSD's Gemini-compatible settings policy, documented shim baseline. |
+| Antigravity | Skills in `skills/gsd-*/SKILL.md`; agents in `agents/`; Gemini-style `settings.json` hooks when installed by GSD | Global `ANTIGRAVITY_CONFIG_DIR` or `~/.gemini/antigravity`; local `./.agents` (canonical, #791) or `./.agent` (legacy, recognized for backward-compat) | GSD owns generated skills/agents/hooks and GSD settings entries only | Public Antigravity install/config docs for this file layout were not stable or complete as of 2026-05-11; installer compatibility therefore uses GSD's Gemini-compatible settings policy, documented shim baseline. Fresh installs write to `.agents/` (the Google-Codelabs-documented form); existing `.agent/` installs continue to be detected and served. |
 | Cursor | Skills in `skills/gsd-*/SKILL.md`; agents in `agents/`; rule references under `rules/`; lifecycle hooks via `hooks.json` (sessionStart + postToolUse, #777) | Global `CURSOR_CONFIG_DIR` or `~/.cursor`; local `./.cursor` | GSD owns generated skills/agents, GSD rule files or references, and GSD-managed `hooks.json` entries (sentinel `gsd-managed:true`); no statusline ownership | [Cursor rules](https://docs.cursor.com/context/rules); [Cursor hooks](https://docs.cursor.com/context/hooks); docs not versioned, checked 2026-06-07 |
 | Windsurf | Skills in `skills/gsd-*/SKILL.md`; agents in `agents/`; rule references under `rules/` | Global `WINDSURF_CONFIG_DIR` or `~/.codeium/windsurf`; local `./.windsurf` | GSD owns generated skills/agents and GSD rule files or references; no hook/statusline ownership | Windsurf public rule docs were source-limited in search results as of 2026-05-11; installer targets the common workspace rules convention `./.windsurf/rules` and must be rechecked before migrations rewrite rules |
 | Augment Code | Skills in `skills/gsd-*/SKILL.md`; agents in `agents/` | Global `AUGMENT_CONFIG_DIR` or `~/.augment`; local `./.augment` | GSD owns generated skills/agents only; no hook/statusline ownership | [Augment Agent Skills](https://docs.augmentcode.com/cli/skills), [Augment IDE skills](https://docs.augmentcode.com/using-augment/skills); IDE skills public beta in VS Code 0.789.0+, checked 2026-05-11 |