Auto-updater prunes the running version's binary, silently breaking all subagent/teammate spawns mid-session

Summary

The auto-updater can prune the currently-running version's binary from ~/.local/share/claude/versions/. The running session survives (the OS keeps the deleted inode alive), but every subsequent subagent/teammate spawn silently fails: the launcher execs children via the parent's versioned binary path, which no longer exists.

Environment

• Platform: macOS (Darwin 25.5.0)
• Session started on version: 2.1.221
• Versions dir at failure time: 2.1.191, 2.1.204, 2.1.222, 2.1.226, 2.1.227, 2.1.228 — 2.1.221 absent (pruned mid-session)
• Long-lived interactive session (multi-day), agent teams enabled (CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=1)

Repro / evidence

Run a long-lived session; let the auto-updater install newer versions and prune the running one. Then spawn a subagent (Agent tool / teammate). The spawn command the launcher constructs:

env CLAUDECODE=1 CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=1 $HOME/.local/share/claude/versions/2.1.221 \
  --agent-id <name>@session-<id> --agent-name <name> --team-name session-<id> \
  --parent-session-id <id> --agent-type general-purpose ... 
env: $HOME/.local/share/claude/versions/2.1.221: No such file or directory

Impact

• From inside the session, Agent tool calls return "Spawned successfully" and the agent simply never runs and never reports — the failure is silent from the model's/user's perspective (it looks like a hung/lazy agent). We lost several hours across multiple days to "agents never started" before catching the exec error by running the spawn command manually.
• Everything else in the session keeps working, which makes it harder to suspect the binary.

Expected

Any of:

1. The updater never prunes the version a live session is running (pid/lockfile check).
2. The teammate/subagent launcher falls back to the stable claude shim (or newest installed version) when the parent's versioned path is missing.
3. At minimum, the spawn failure surfaces as a loud error in the session instead of a silent no-op after "Spawned successfully".

Workaround

ln -s ~/.local/share/claude/versions/<newest> ~/.local/share/claude/versions/<pruned-running-version> — spawns recover immediately; minor parent/child version skew appears harmless.