[Feature] Background watch notifications (--notify)

[hahwul]

Since AI software engineering sessions can take some time to run, it is inconvenient to keep looking at the terminal. Having notification integrations would greatly improve the feedback loop.

Example Usage:

# Triggers a macOS system notification when session ends or requests feedback
cjules watch <id> --notify desktop

# Send notification to a webhook (Slack/Discord)
cjules watch <id> --notify slack

Details:

• Should trigger when the session state changes to COMPLETED, FAILED, or AWAITING_USER_FEEDBACK.
• Configuration for webhook URLs could be saved in ~/.config/cjules/config.yml.