Hand-off engine: push agent branch and open one structured PR per repo

[Ryan-Atkinson87]

Spec: §5.4 (hand-off — the only push), §4.2 step 6, §7.3.

At wave completion, push the agent branch once per repo and open one PR per repo from the agent branch to the integration branch, with the structured body the spec defines. This is the only push to GitHub in the whole wave.

Acceptance criteria

• Pushes the agent branch to origin exactly once per repo (no mid-wave pushes), using the GitHub client (GitHub API client for hand-off (PAT-scoped: push agent branch + PRs only) #13).
• Opens one PR per repo from the agent branch to branches.integration (default dev).
• The PR body includes, per §5.4 step 3: a Summary; per-issue notes referencing issues as Issue: #N (never Closes); a pre-checked list of engine-verified items (tests/lint/typecheck from Test/lint/typecheck gate runner with structured per-repo results #12, plus other programmatic checks) rendered as ticked boxes; a human-only checklist of open checkboxes (visual / responsive / accessibility / production-only integrations), omitted entirely when empty; a Parked blockers section; and the "CI must pass before merging" note.
• After the human merges the PR, the remote agent branch is deleted (§5.4 step 5); the next wave creates its own fresh branch.
• Human notification (step 4) and the Social Media Context Notion update (step 6) are explicitly out of scope here and deferred to Phase 5 (Trackers + notifications) — leave a clear extension point, not a stub that silently does nothing.
• Unit tests cover PR-body assembly (each section, including the omitted human-checklist case and the Issue: #N form) and the push-once / delete-after-merge sequencing, with the GitHub client mocked.
• Lint, format, and typecheck pass.

Depends on: #10, #12, #13

Ordering: last in the phase; needs the agent branch (#10), gate results (#12), and the GitHub client (#13).