autopilot: notify step

[Angelmmiguel]

Summary

Add the notify step that posts messages to GitHub (issue/PR comments) and records activity to memory.

Requirements

• Receives notify actions from coordinator, resolver, or pusher
• AI composes GitHub comments adapted to channel visibility (public vs private)
• Posts via GitHub API (gh CLI)
• Rover footer marker required in all posts for self-detection
• Writes to memory (records "action taken" with PR URL, files changed)
• Also used for clarification pattern: coordinator produces notify to ask questions
• Prompt: steps/prompts/notify-prompt.md (~3.5KB system prompt)
• Terminal step (no further actions produced)
• Add tests for comment composition, footer marker inclusion, memory recording

References

• Reference: rover-autopilot/packages/cli/src/lib/autopilot/steps/notify.ts (~14KB)
• Prompt: rover-autopilot/packages/cli/src/lib/autopilot/steps/prompts/notify-prompt.md