cmux claude-hook stop: TabManager not available error on session end

Description

When a Claude Code session ends inside cmux (v0.62.2), the Stop hook consistently fails with:

``
Ran 1 stop hook
cmux claude-hook stop
Stop hook error: Failed with non-blocking status code: Error: unavailable: TabManager not available
``

## Environment

• cmux version: 0.62.2
• OS: macOS Sequoia 26.3 (Darwin 25.3.0)

## Root Cause Analysis
The cmux wrapper injects a Stop hook (cmux claude-hook stop) that connects to cmux via Unix socket
and tries to access the internal TabManager. The TabManager is already disposed during shutdown.

## Impact
Non-blocking, no functional impact. But the error is shown on every session end (noisy).

## Expected Behavior
Gracefully handle TabManager unavailability during shutdown.

## Steps to Reproduce

1. Open cmux
2. Start a Claude Code session
3. End the session
4. Observe the stop hook error