/retro should persist recommendations and detect follow-through in subsequent runs

## Problem

`/retro` generates "3 Things to Improve" recommendations as prose output but does not save them to the JSON snapshot in `.context/retros/`. On the next run, it loads the prior snapshot for trend comparison but has no structured data about what it recommended.

This means the retro cannot detect when current-week work addresses prior recommendations. In practice, a week spent acting on retro feedback (security hardening, dependency bumps) gets mischaracterized as generic "fix ratio is high" instead of "2 of 3 prior recommendations addressed."

## Proposed fix

1. Add a `recommendations` array to the Step 13 JSON schema:

```json
{
"recommendations": [
{"category": "security", "text": "Run pip-audit on a schedule"},
{"category": "testing", "text": "E2E fixture reliability pass"},
{"category": "architecture", "text": "Extract html_generator rendering modules"}
]
}
```

2. In Step 12 (Load History & Compare), cross-reference prior `recommendations` against current-week commit messages and file changes. Report which were addressed, which are still open.

3. Add a "Retro Follow-Through" section to the Step 14 narrative, before "3 Things to Improve":

```
Retro Follow-Through (from May 7):
[x] Security hardening — 7 PRs shipped (#1301, #1315, #1318, ...)
[x] Dependency audit — lxml, Authlib, idna bumped (#1352, #1304)
[ ] E2E fixture reliability — still fragile (#1272, #1332 were patches, not a root fix)
```

## Why it matters

The retro loop is a forcing function. If it cannot see its own prior output, it treats retro-driven work as unexplained and generates misleading narratives about what the week was actually about.