Handover doc

Create or update a handover doc for the current repo.

Location

• If repodocs/prompts/ exists -> use it.
• Else use docs/prompts/.

Filename

• YYYY-MM-DD-handover-<topic>.md
• <topic> is auto-slugged on creation, using best available context:
  • Prefer PR title (if PR exists for current branch).
  • Else a short task/context summary (e.g., feature name, incident, or subsystem).
  • Else latest commit subject.
• If the user requests a specific slug/topic, honor it.
• On update: do not rename; update existing file unless the user requests a new file.