feat(cli): add Antigravity CLI as a flat host for skills

[tkman59]

🎯 What & why

Teams moved from Gemini CLI to Antigravity CLI (agy) get no AIDD install. This adds an isolated antigravity flat profile that writes AIDD skills under .agents/skills/, from aidd setup --ai antigravity and from aidd translate --to antigravity --as flat. First of four PRs for #511: skills only; the memory hook, agents and docs follow.

🛠️ How it works

• Profile in cli/src/contexts/tools/domain/profiles/antigravity/, modelled on Kilo: skills supported, every other artifact supported: false with a reason, hooks refused until the next phase.
• Skills sit one level deep. Measured on agy 1.2.14 (2026-10-05): a probe at .agents/skills/flat-probe/SKILL.md logs expanded slash command "flat-probe" (skill); one at .agents/skills/aidd-probe/nested-probe/SKILL.md logs no expansion. So Antigravity can't use the skills/<plugin>/<skill>/ tree Kilo and OpenCode get.
• A flat profile now declares its skill layout: nested (default, unchanged for every existing tool) or single-level, skills/<plugin>-<skill>/ (plugins-capability.ts:136, set at antigravity/profile.ts:45). The flat install reads it when writing (content-translator.ts:353) and when matching a built file back to its plugin (built-tree-materialization-translator.ts:185). Before that, setup installed 0 skills from the one-level tree.
• Local-path install no longer emits commands for a tool without a commands capability (content-translator.ts:257); it wrote a stray .agents/commands/ for Antigravity. No other tool's output changes.
• Registration everywhere tool-addition-cost.arch demands, a tools-antigravity mutation scope (82.5), and a { tool: antigravity, mode: flat } row in the build-per-tool matrix.

🧪 How to verify

• cd cli && pnpm test: 6818 passed.
• Golden: golden.json gains only the antigravity:flat cell; no existing cell changes.
• In an empty git repo: aidd setup --source local --path <framework> --ai antigravity --plugins all --yes writes 51 .agents/skills/*/SKILL.md, all one level deep. Then, logged in to agy: agy -p "/aidd-context-11-explore" --add-dir . logs expanded slash command "aidd-context-11-explore" (skill) in ~/.gemini/antigravity-cli/log/cli-*.log. Read the log, not the turn: print mode hangs after answering.
• tests/e2e/clean-antigravity-foreign-file.e2e.test.ts: a user file under .agents/skills/ survives aidd clean.

⚠️ Heads-up

• No CI runtime smoke, unlike Kilo: an agy turn needs a Google login. CI covers the golden cell and the matrix build only.
• Known limitation, tracked outside feat(cli): support Antigravity CLI as a flat host #511: Codex and Antigravity both write .agents/skills/; uninstalling one deletes plugin files the other still records.
• Plugin-prefix matching: a single-level skill belongs to a plugin by its <plugin>- prefix, so a future plugin named aidd-dev-x would match aidd-dev. Flat agents already work this way.
• Local pre-commit: scripts-tests was excluded for this commit. check-written-file-hook.test.js writes and deletes a file in cli/src while comments-name-files-that-exist.test.js reads that directory in parallel (ENOENT .hook-probe-<pid>-broken.ts). This happens locally only, and the same suite passes run serially (549/549). CI skips the writer because Biome isn't installed there.

🔗 Linked issue

Refs #511

✅ I certify

• I DO CERTIFY I READ EACH LINE OF THE PULL REQUEST BECAUSE I AM A SOFTWARE ENGINEER, NOT A AI PUPPY.

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