fix(chat): restore first-turn fly-in after workspace change

[tomtom-bot7]

Summary

• Soft session swap after /change (fork) left ChatView mounted with loading flags that made hasVisibleConversation true, docked the composer, and skipped FirstTurnFlight — and stuck follow-up transitions until Cmd+R.
• Gate first-turn on getCachedItemCount(sessionId) === 0 instead of !hasVisibleConversation().
• On soft sessionId change, remount-equivalent reset: abort flights, clear handoffs, set firstTurnFlightDone from cache, center empty idle sessions; stop docking solely for isLoading.
• /change centers composer before soft goto.

Test plan

• New chat → /change to another workspace → send first message → hero fly-in + dock runs
• Same path: send a second message → follow-up transition works without Cmd+R
• Empty session load keeps centered composer / EmptyChatState until real content or first-turn
• Session with existing messages still docks; no spurious FirstTurnFlight
• vitest on conversation-controller / thread-visibility / start-chat / composer-focus / first-turn-flight (60/60 locally)

Reviewed two-axis by tomtom.bot40 @ c0ddcf2.

[tomtom-bot40]

Standards

Focused fix across ChatView + conversation-controller + /change. No blocking smells. Tests updated for empty-cache firstTurn and remount-equivalent bind.

Spec

Matches the agreed Spec for soft workspace/session swap:

1. firstTurn via getCachedItemCount === 0 (not hasVisibleConversation / loading)
2. Soft sessionId swap is remount-equivalent (abort flights, clear handoffs, firstTurnFlightDone = cachedCount > 0, empty idle → empty snapshot + centerComposer())
3. Do not dock solely for isLoading
4. /change centers before soft goto

Also covers follow-up transitions stuck until Cmd+R by resetting composer phase on soft swap.

Standards: pass · Spec: pass · pre-PR two-axis sign-off stands

[Tomlord1122]

LGTM