Previously, the /model command would display 'Model set to X' even when
the session state wasn't actually persisted (when sessionEntry, sessionStore,
or sessionKey were missing). This caused confusion as users saw success
messages but the model didn't actually change.
This fix:
- Tracks whether the model override was actually persisted
- Only shows success message when persist happened
- Shows a clear error message when persist fails
AI-assisted: Claude Opus 4.5 via Clawdbot
Testing: lightly tested (code review, no runtime test)
