#3322 repo init --help omits pi and antigravity from the built-in --tool list

What happens

swamp repo init --help says:

Built-in: amp, claude, cursor, opencode, codex, copilot, kiro, none.

But VALID_AI_TOOLS in src/cli/ai_tool_parser.ts also includes pi and antigravity, and both work: repo init --tool pi writes AGENTS.md and .pi/extensions/swamp-audit.ts; repo init --tool antigravity writes AGENTS.md and .agents/hooks.json.

Why it matters

Someone reading the help concludes pi and antigravity are unsupported. It already misled a docs pass into proposing their removal from the manual, which still documents both.

Suggested fix

Build the help's Built-in list from VALID_AI_TOOLS rather than a hand-written string, so the two cannot drift again. Check any other command that lists --tool values (repo upgrade, audit) the same way.

Repro

Binary 20261012.200126.0-sha.3b7bb2ee:

swamp repo init --help | grep -A3 -- --tool
swamp repo init --tool pi --no-vault        # succeeds
swamp repo init --tool antigravity --no-vault   # succeeds