A terminal migrates a chat another process holds before it knows the chat is held #2063

Found while fixing #2046. When a terminal opens a chat that another process on the same computer holds, openV3Agent (cmd/codeaf/chatv3.go) runs v3Migrated on it before the session open learns the journal is locked. #2046 stopped the sync drive from acting in that window; the migration write itself still happens.

Window: from the terminal's open until open() returns ErrSessionLocked.

Harm: not shown yet. Possible effects are a write into another process's live journal or a migration racing its writer.

Repro idea: two processes on one codeaf home (the desktop app's engine holding a chat, a terminal codeaf chat --session opening the same chat); snapshot the journal and the held chat's state before and after the terminal opens and quits, repeated, and with a chat that needs migrating.

🤖 Generated with Claude Code