process-gap(pr-manager): post-merge audit artifacts (pr-description.md + pr-review.md) left uncommitted

## Class of failure

The pr-manager agent's 9-step PR process (per `per-story-delivery.md` and `workflows/code-delivery.lobster`) generates two valuable audit-trail artifacts during execution:

1. **Step 1: populate-pr-description** → writes `.factory/code-delivery/STORY-NNN/pr-description.md` (PR body, convergence summary, deferrals)
2. **Step 5: review-convergence** → writes `.factory/code-delivery/STORY-NNN/pr-review.md` (pr-reviewer findings, verdict, convergence cycles)

Neither file is committed to `factory-artifacts` by default. They linger as uncommitted artifacts in the worktree until manually committed post-merge (often hours later — or never if the human doesn't notice).

## Pattern observed

In an external pilot project, after 2 successful PR merges in the same cycle:

```
$ cd .factory && git status --porcelain
?? code-delivery/STORY-A.AA.XXX/
?? code-delivery/STORY-B.BB.YYY/
```

3 audit-trail artifacts (2× pr-description.md + 1× pr-review.md, total ~50 KB) sat uncommitted for hours until the orchestrator caught it via factory-health.

When a session resumes from a clean state, the absence of these artifacts in `factory-artifacts` history loses:
- The exact PR body language the pr-manager generated (useful for post-mortem)
- The pr-reviewer's findings + convergence trajectory (useful for retrospective)
- The decision rationale captured in the PR body (deferrals, accepted trade-offs)

## Root cause

The pr-manager 9-step process doesn't have an explicit "commit + push these artifacts" sub-step at step 9 (post-merge). Step 9 currently focuses on worktree removal + local branch deletion.

## Proposed remediation

Extend per-story-delivery step 9 (post-merge):

```
Step 9.x: pr-manager — commit + push:
.factory/code-delivery/STORY-NNN/pr-description.md
.factory/code-delivery/STORY-NNN/pr-review.md
(if either was written during steps 1+5)
to factory-artifacts with signed commit message:
"chore(audit): pr-manager artifacts for STORY-NNN (PR #X merged)"
```

Optionally enforce via factory-health: warn if `.factory/code-delivery/` has untracked files.

## Labels

- `process-gap`
- `pr-manager`
- `per-story-delivery`
- `audit-trail`