Concurrent thread resume from two terminals causes shared task state and data loss

Bug Description

When the same conversation thread is resumed simultaneously from two separate terminal instances (e.g., claude --resume in Terminal A while Terminal B is already running the same thread), the task lists merge across both sessions. When one terminal is quit, the other terminal's in-flight background agents are orphaned and their work is lost with no way to recover.

Steps to Reproduce

1. Open Terminal A, start a Claude Code session — begin working on Task A with background agents
2. Open Terminal B, run claude --resume (or resume the same thread from the session picker)
3. In Terminal B, start working on an unrelated Task B — create tasks, launch subagents
4. Observe: Terminal A's task list now shows Terminal B's tasks mixed in (and vice versa)
5. Quit Terminal B (Ctrl+C or close the terminal)
6. Observe: Terminal A still shows the mixed task list. Terminal B's subagents are killed silently. No way to resume Terminal B's session — it does not appear as a separate resumable thread.

Expected Behavior

One of:

• Thread locking: Second terminal should be warned that the thread is already active in another process and either block resume or fork a new session
• Session isolation: Tasks and subagents should be namespaced per terminal process, not shared at the thread level
• Graceful recovery: Orphaned subagents should be resumable, or at minimum their partial output should be persisted

Actual Behavior

• Task lists are globally shared per thread ID across all terminal processes
• Tasks from both sessions are interleaved with no indication of which terminal owns which
• Background agents launched by Terminal B are silently killed when Terminal B exits
• Terminal B's conversation is merged into Terminal A's session JSONL — there is no separate session file to resume
• The work done by Terminal B's subagents (file reads, analysis, in-progress code changes) is permanently lost

Impact

• Users who habitually open multiple terminals for different workstreams on the same project can accidentally corrupt session state
• Background agent work (which can represent significant LLM cost and time) is silently discarded
• No warning or error is surfaced — the failure mode is silent data loss

Environment

• Claude Code version: 2.1.83
• Platform: WSL2 (Ubuntu 22.04.5 LTS, kernel 6.6.87.2-microsoft-standard-WSL2)
• Node: v24.1.0
• Terminal: Two separate zsh terminal instances in the same WSL2 distro