Bug: /resume does not update sessions/{pid}.json sessionId

Summary

When using /resume inside an already-running Claude Code session to switch to a different conversation, the ~/.claude/sessions/{pid}.json file is not updated with the new session ID. This causes external tools and hooks to have an incorrect view of which session a process is operating on.

Steps to Reproduce

1. Open Window A, start Claude Code → creates session SA, writes sessions/{pidA}.json with sessionId: SA
2. Open Window B, start Claude Code → creates session SB, writes sessions/{pidB}.json with sessionId: SB
3. In Window B, type /resume and select session SA
4. Inspect sessions/{pidB}.json → still shows sessionId: SB instead of SA

Impact

This causes JSONL message chain forks. Since sessions/{pid}.json doesn't reflect the actual session being operated on:

• Two Claude Code processes can silently operate on the same session simultaneously
• The JSONL file develops forked message chains (two children with the same parentUuid, both with isSidechain: false)
• claude --resume later only follows one branch of the fork, losing messages from the other branch
• SessionStart hooks cannot reliably detect and prevent duplicate processes on the same session

Evidence

The JSONL uses uuid + parentUuid to form a message chain (tree structure). When two processes write to the same JSONL, fork points appear:

# Same parentUuid referenced by two different children = fork
$ jq -r '.parentUuid' session.jsonl | sort | uniq -c | sort -rn | head
   2 dc26f80c-bbda-409a-8a8c-5d65bfb2bd5b
   2 939c92b3-8870-4b35-99bd-339e045fb176
   2 897833f0-d882-4422-8473-b3ad1a3263d5
   ...

We observed 6 fork points in a real session where a second process resumed the same conversation.

Expected Behavior

When /resume switches to a different session, update ~/.claude/sessions/{pid}.json to reflect the new sessionId. This would allow:

1. External tools to correctly identify which session a process is operating on
2. SessionStart hooks to detect and prevent concurrent access to the same session
3. Prevention of silent JSONL message chain forks

Workaround

Currently, users must manually /exit the original session before resuming it from another window or tool. A SessionStart hook can detect and kill conflicting processes, but only for claude --resume from the shell — not for /resume within an existing session, because the session file is stale.

Environment

• Claude Code version: 2.1.81
• OS: macOS (Darwin 25.3.0)