claude --resume allows concurrent access to same session from multiple terminals

When resuming a session with claude --resume <id>, there is no check for whether another claude process already has that session open. Two terminals can simultaneously write to the same session JSONL file.

Consequences

• Interleaved writes to the session file (no file locking)
• Both instances see each other's entries, leading to incoherent context
• Potential file corruption from partial/concurrent writes

Steps to reproduce

1. Start a Claude Code session in Terminal 1
2. In Terminal 2, run claude --resume <same-session-id>
3. Both terminals are now active on the same session file

Expected behavior

Claude should either refuse to resume a session that's already open (with a message like "session already open in PID X") or use file locking to prevent concurrent writes.