Allow ACP session settings to be changed offline and replayed on reconnect

[cxxxxxn]

Problem

ACP session settings currently require a live connection. If the agent is unavailable, changing a cached model, mode, or config option fails and the optimistic selection is reverted.

Expected behavior

Treat selector changes as durable per-thread intent:

• Allow changing cached settings while the ACP agent is offline.
• Persist the requested selection before attempting the live RPC.
• Replay pending selections when the session reconnects.
• Preserve intent after transient connection failures.
• Clear intent only when the agent definitively rejects the value.
• Show a pending synchronization state.
• Keep selectors unavailable only when no cached catalogue exists.

Existing foundation

The ACP driver already persists per-thread selections and replays them when opening or resuming a session. However, the current set-RPC path records a selection only after the live control operation succeeds.

Acceptance criteria

• Offline model selections survive reload.
• Pending selections are applied before the next prompt after reconnect.
• Transient failures do not erase user intent.
• Unsupported selections are removed with an actionable message.
• Existing online changes remain immediate.
• Tests cover persistence, reconnect replay, rejection, and missing catalogues.