fix: gate codex-acp until packaging + rediscovery are ready

[c-h-]

Follow-up to Charlie's blocker comment on #127: #127 (comment)

What this does

• removes codex-acp from the public adapter registry for now
• returns a targeted error if someone tries --adapter codex-acp
• updates README install/docs to state which adapters are actually public today
• adds an ADR rollout-boundary note so the architecture direction does not overclaim current shipping status
• adds a CLI test covering the targeted codex-acp rejection path

Why

PR #127 merged the ACP Phase 1 scaffolding, but the public codex-acp surface is still missing two merge blockers Charlie called out:

1. packaged/distributed bridge story
2. discover-first rediscovery / reattach after fresh CLI process / daemon restart

Until those exist, the safest thing is to keep the ACP work internal/experimental and point users to codex.

Validation

• npm test
• npm run typecheck
• npm run build