fix: propagate non-zero exit codes from deploy client

[LongOddCode]

Summary

Fixes #665

The deploy() function spawned the deploy client binary (StaticSitesClient) but never awaited the child process. The close event handler ran in a fire-and-forget callback, so a non-zero exit code was silently ignored and the function always resolved — making CI/CD pipelines believe deployment succeeded even when it failed.

Changes

src/cli/commands/deploy/deploy.ts

• Wrap the spawn() call and its event handlers in new Promise and await it
• Reject the promise on non-zero exit codes with a descriptive error message
• Reject on signal kills (e.g., SIGTERM)
• Reject on spawn errors (e.g., ENOENT when binary not found)
• Add a settled guard to prevent double-resolution edge cases
• Re-throw caught errors in the catch block (previously swallowed silently)
• Moved cleanUp() out of the close handler into the existing finally block

src/cli/commands/deploy/deploy-exit-code.spec.ts (NEW)

• 6 focused unit tests for exit code propagation:
  • Rejects on non-zero exit code
  • Resolves on exit code 0
  • Rejects when killed by signal
  • Rejects on spawn error events
  • Verifies cleanUp() is called after failure
  • Verifies logGitHubIssueMessageAndExit() is called on failure