fix: release shells mutex before spawn_subshell to prevent deadlock

[ibash-corpusant]

Problem

select_shell_desc() holds the global shells mutex while calling spawn_subshell(), which calls wait_for_startup(). If wait_for_startup blocks (e.g. due to a version mismatch between client and daemon, or a shell that fails to start), the mutex is held forever, deadlocking the entire daemon. All operations (list, attach, detach, kill) that need the shells mutex hang indefinitely.

This was observed in production when a daemon that had been running for 2+ days encountered a client/daemon binary mismatch after a package update. The wait_for_startup sentinel handshake hung, and from that point on, every shpool list, shpool attach, etc. blocked forever.

Fix

Refactor select_shell_desc() into three phases:

Phase	Lock held?	What it does
Phase 1	✅ Yes	Determine whether to create a new session or reuse existing
Phase 2	❌ No	Call spawn_subshell() (which may block in wait_for_startup)
Phase 3	✅ Yes (briefly)	Insert session into table, return Arc references

The critical change: spawn_subshell() is now called after the shells mutex is released. If wait_for_startup blocks, only the attach thread is affected — list, kill, detach, and other attach requests proceed normally.

Also adds a 30-second timeout to wait_for_startup() as defense-in-depth. Previously it was an infinite loop.

Regression Test

New test (deadlock.rs) that:

1. Starts a daemon with a fake shell that never produces the startup sentinel
2. Spawns an attach (which hangs in wait_for_startup)
3. Asserts that shpool list still completes within 5 seconds

Before fix: DEADLOCK DETECTED — list times out after 5s
After fix: list returns immediately, test passes in 3.2s

Test Results

All existing tests pass (51 passed, 1 pre-existing prompt_prefix_fish failure unrelated to this change).

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[ethanpailes]

Please explain the extent to which you used AI to generate this PR. It will help me know how closely I have to read it while reviewing.