Add skill-routing integration tests to PR CI pipeline

## Background

PR #1210 introduced a skill-routing regression (#1599) that went undetected for **20 days** because integration tests only run on a nightly schedule, not on PRs.

## Problem

- `test-all-integration.yml` runs only on `schedule` (nightly cron) and `workflow_dispatch` (manual)
- `test-all-skills.yml` runs on PRs but only runs unit/trigger tests, which don't test LLM-based routing
- The failing scenario (generic deploy prompt + codebase context → wrong skill wins) can only be caught by integration tests

## Proposal

Add a **lightweight subset** of integration tests to the PR CI pipeline:

1. **Skill-routing smoke tests** — For each skill that has `PREFER OVER` or `DO NOT USE FOR` clauses, run the integration test scenarios that validate correct routing
2. **Triggered only when skill files change** — Use `paths:` filters to only run when `plugin/skills/**` or `tests/**/integration.test.ts` are modified
3. **Parallel matrix** — Run only affected skills, not the full matrix

### Example workflow trigger

```yaml
on:
pull_request:
paths:
- 'plugin/skills/**'
- 'tests/**/integration.test.ts'
```

## Impact

This would have caught #1599 at PR #1210 review time, preventing the 20-day detection delay.

## Related

- Root cause: #1599
- Fix: #1644

Start by comparing the triggers and test scope in test-all-integration.yml and test-all-skills.yml. Review the integration tests under tests/**/integration.test.ts and the skill files under plugin/skills/**, then determine how to select affected skills in a parallel PR matrix. Done means routing smoke tests run for relevant changes, while unrelated pull requests retain the existing CI behavior.

Written by the indexing model from the issue text.