[#256] feat: implement composes checkpoint + publish-pr via handoff-only subagent

.claude/skills/pair-process-implement/post-review-merge.md

@@ -43,6 +43,15 @@ Disclosed from [SKILL.md](./SKILL.md) Phase 4 — only reached when `/pair-proce
 3. **Act**: Check parent initiative — if ALL epics in the initiative are Done, update initiative status to "Done".
 4. **Verify**: Story and parent hierarchy updated recursively.
 
+### Step 4.5: Clean Up the Checkpoint
+
+The checkpoint's lifecycle ends at merge — it exists only to survive context resets and the review/fix loop, both of which are over once the story is Done.
+
+1. **Check**: Does `.pair/working/checkpoints/<story-id>.md` exist?
+2. **Skip**: If no checkpoint file exists (e.g. `/pair-capability-checkpoint` was not installed), nothing to clean up.
+3. **Act**: Remove `.pair/working/checkpoints/<story-id>.md` so finished-story state never lingers.
+4. **Verify**: The checkpoint file is gone (checkpoint lifecycle: written at the closing phase — Step 3.2 — cleaned up here at merge).
+
 ## Output Format (merge)
 
 ```text

apps/website/content/docs/developer-journey/execution.mdx

@@ -51,13 +51,14 @@ Every new module file must have a corresponding unit test file (1:1 mapping). Mo
 
 With commit-per-task, the developer gets a checkpoint between every task — the AI presents a summary and waits for confirmation before committing.
 
-### PR Creation
+### Closing Phase: Checkpoint + PR
 
-After all tasks are complete, the skill:
-1. Pushes the feature branch
-2. Creates a PR following the team's [PR template](/docs/reference/guidelines-catalog)
-3. Links the PR to the story issue
-4. Marks the PR as ready for review
+After all tasks are complete, the skill's closing phase splits the handoff from the publish so the PR is always built on a clean context:
+
+1. **Writes a checkpoint** (`/pair-capability-checkpoint` in write mode) — the boundary artifact capturing story, branch, tasks done, decisions, and remaining todos. This is the handoff document the PR is built from, not this session's memory.
+2. **Publishes the PR via a handoff-only subagent** — spawns a fresh, anonymous subagent whose entire prompt is the checkpoint, which resumes from it and runs `/pair-capability-publish-pr`. That skill runs the quality gate, pushes the branch, creates or updates one PR from the [PR template](/docs/reference/guidelines-catalog), links it to the story, copies the classification tags, and marks it ready for review.
+
+If subagent spawning is unavailable, the skill degrades to running `/pair-capability-publish-pr` inline in the same session (checkpoint still written) and notes the degradation. Re-invoking `/pair-process-implement` on an interrupted story reads the checkpoint and resumes from the first pending task — completed tasks are never repeated, and `publish-pr` updates the existing PR rather than opening a second one.
 
 ## Step 9: Code Review
 
@@ -106,7 +107,9 @@ After merge, the skill updates the status hierarchy:
     │
     ├── Task 1: RED → GREEN → REFACTOR → commit
     ├── Task 2: RED → GREEN → REFACTOR → commit
-    └── Task N: ... → commit → PR created
+    ├── Task N: ... → commit
+    │
+    └── Closing: checkpoint (write) → fresh subagent resumes → publish-pr → PR ready
     │
 /pair-process-review #pr-number
     │

apps/website/content/docs/reference/skills-catalog.mdx

@@ -26,7 +26,7 @@ Process skills orchestrate multi-step workflows by composing capability skills.
 | **plan-stories** | `/pair-process-plan-stories` | Slices an epic into user stories via vertical slicing and INVEST validation, each sized for one sprint. | `/pair-capability-write-issue` |
 | **refine-story** | `/pair-process-refine-story` | Refines a user story from Draft to Ready — the single Draft→Ready path (D24): phase 0 grill(sync), Given-When-Then acceptance criteria, map-subdomains/map-contexts scoped analysis, classify matrix, sprint readiness. Not for sizing an already-refined story (use estimate). | `/pair-capability-grill`, `/pair-capability-write-issue`; `/pair-capability-map-subdomains`, `/pair-capability-map-contexts`, `/pair-capability-classify` (optional) |
 | **plan-tasks** | `/pair-process-plan-tasks` | Breaks a refined user story into implementation tasks — checklist, dependency graph, AC-coverage table — added to the story body; no separate task issues. | `/pair-capability-write-issue` |
-| **implement** | `/pair-process-implement` | Implements a refined user story task-by-task via a 5-step cycle per task (context, branch, implementation, quality, commit); opens one PR when the story completes. | `/pair-capability-verify-quality`, `/pair-capability-record-decision` |
+| **implement** | `/pair-process-implement` | Implements a refined user story task-by-task via a 5-step cycle per task (context, branch, implementation, quality, commit); at the closing phase it writes a checkpoint and publishes one PR through a handoff-only subagent, resuming from the checkpoint when re-invoked on an interrupted story. | `/pair-capability-verify-quality`, `/pair-capability-record-decision`, `/pair-capability-checkpoint`, `/pair-capability-publish-pr` |
 | **review** | `/pair-process-review` | Reviews a pull request through 6 sequential phases (5 review + optional merge with parent cascade) to decide whether it merges. Not a quick build/test sanity check (use verify-quality). | `/pair-capability-classify`, `/pair-capability-verify-quality`, `/pair-capability-verify-done`, `/pair-capability-record-decision`, `/pair-capability-analyze-debt`, `/pair-capability-assess-security` (required); `/pair-capability-verify-adoption`, `/pair-capability-assess-stack`, `/pair-capability-execute-manual-tests` (optional) |
 
 ## Capability Skills
@@ -118,7 +118,7 @@ All `analyze-*` skills are **output-only report producers**: they analyze and re
 | Skill | Command | Description |
 | ----- | ------- | ----------- |
 | **checkpoint** | `/pair-capability-checkpoint` | Writes and resumes a self-contained progress checkpoint (story, branch, tasks done, decisions, remaining todos) in `.pair/working/checkpoints/`, so work survives a context reset. Invoke directly to save or resume progress. |
-| **publish-pr** | `/pair-capability-publish-pr` | Publishes a completed story branch as a pull request: runs the quality gate (HALTs on red), creates or updates ONE PR from the pr-template with conditional sections (Services to Release, Screenshots) filled only when pertinent, copies the story's classification tags, marks it ready-for-review, and updates the board state. Standalone — driven by a handoff/checkpoint; reads the WoW `git-workflow` (base-branch, squash) and `code-host`. Composed by a future closing phase of `/pair-process-implement` (not yet wired); reused by hotfix and automation loops. Never merges. |
+| **publish-pr** | `/pair-capability-publish-pr` | Publishes a completed story branch as a pull request: runs the quality gate (HALTs on red), creates or updates ONE PR from the pr-template with conditional sections (Services to Release, Screenshots) filled only when pertinent, copies the story's classification tags, marks it ready-for-review, and updates the board state. Standalone — driven by a handoff/checkpoint; reads the WoW `git-workflow` (base-branch, squash) and `code-host`. Composed by the closing phase of `/pair-process-implement` (invoked in a handoff-only subagent so the PR is built on a clean context); reused by hotfix and automation loops. Never merges. |
 
 ## Skill Properties