Fix Resume button not showing for crashed spawned sessions

[0xabstracted]

Summary

• Fixes a race condition where the Resume button doesn't appear on sessions that crash immediately after spawning (exit code 1)
• When a spawned agent crashes before invoke('create_session') returns, the pty-exit handler can't find the session in the store yet, so it skips the server status update — leaving the maestro session stuck at spawning
• The Resume button only renders for ['completed', 'stopped', 'failed', 'idle'], so spawning sessions never show it

Changes (useSessionStore.ts)

1. applyPendingExit now sends status: 'stopped' to the maestro server when consuming a pending exit that has a maestroSessionId
2. handleSpawnTerminalSession now calls applyPendingExit on the new session before adding it to the store, catching the race immediately

Test plan

• Spawn a session that crashes immediately (e.g., bad working directory or invalid command) — verify Resume button appears
• Normal session lifecycle (spawn → work → complete) — verify Resume button still works
• App restart with exited sessions — verify sessions restore correctly
• Verify no duplicate server status updates for sessions that exit normally (pty-exit handler + applyPendingExit are idempotent since both send stopped)

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