[STORY] US-042: Implement Checkpoint 2 (PR review) DM flow

[carloshumbertoreyesortiz]

Story US-042

Parent epic: E-09 — Slack Intake Bot
Effort: M (5 pts)
Priority: P2 (Medium) — untriaged baseline; re-triage as needed
Impl guide: §12.5
Depends on: US-041

User story

As a reviewer, I want Checkpoint 2 (PR review) delivered as a DM with the PR link and summary, so that I can review the agent's PR without leaving Slack.

Acceptance criteria

• DM sent to the assigned reviewer with PR URL, diff summary, and provenance digest
• Reviewer can jump straight to the GitHub PR from the message
• Approve/request-changes action recorded and reflected on the run
• Falls back gracefully if the reviewer has DMs disabled

Parent epic: #9