Fix agent shutdown hanging on Ctrl+C

[gricha]

Summary

• Fixes the issue where perry agent run hangs on Ctrl+C, requiring double Ctrl+C to exit
• Adds proper graceful shutdown with a 3-second timeout fallback
• Cancels background image pull timers on shutdown
• Force-closes all HTTP keep-alive connections immediately

Changes

src/agent/run.ts:

• Added isShuttingDown flag - first Ctrl+C triggers graceful shutdown, second Ctrl+C force-exits
• Call stopEagerImagePull() to cancel background timers
• Call server.closeAllConnections() to force-close lingering HTTP connections
• Added 3-second force exit timeout (with .unref() so it doesn't keep the event loop alive)

src/docker/eager-pull.ts:

• Added AbortController to signal cancellation of pending retries
• All setTimeout calls now use .unref() so they don't prevent process exit
• Added stopEagerImagePull() export to abort image pull attempts

Behavior

1. Single Ctrl+C: Logs "Shutting down...", closes all connections immediately, exits cleanly
2. If graceful shutdown hangs: Force exits after 3 seconds
3. Double Ctrl+C: Force exits immediately