Add --watch mode for continuous evaluation

[aviralgarg05]

Summary

Add a --watch mode to the CLI that re-runs evaluations when source files change.

Tasks

• Add watchfiles or similar as an optional dependency.
• Implement agentunit eval --watch that monitors tests/ and src/ for changes.
• Re-run the specified scenario(s) on file save.
• Print a summary after each run.

Acceptance criteria

• agentunit eval scenarios/ --watch works.
• Graceful exit on Ctrl+C.
• Documentation updated in CLI help.