[Bug]: Edit from here is hidden on Grok and Antigravity and offered on Muse, unlike what the server accepts#30137

What happens

Whether a client offers Edit from here comes from a hand-set supportsConversationRollback flag in the provider status, while the server decides from the adapter capabilities (ensureRollback in CommandPolicy.ts). The two disagree:

• Grok (provider-grok/src/server/status.ts) and Antigravity (AntigravityProvider.ts) set the flag to false, so the web hides Edit from here, although the server accepts the rollback.
• Muse sets no flag, so the web shows Edit from here, and the server refuses it because Muse cannot roll back.

Steps to reproduce (main cefb544af3)

1. Open a finished Grok or Antigravity thread on web. No sent prompt offers Edit from here.
2. Open a finished Muse thread. Edit from here is offered, and choosing it fails.

Expected

Edit from here is offered exactly where the server accepts the rollback, from one rule, so a new adapter cannot drift.

I have a fix that derives the status flag from the adapter capabilities with the same predicate the policy enforces, and removes the hand-set flags. It builds on the fix for #30136, which lets ACP rollbacks through the policy. I can open a PR if you want one.