feat: QA test plan tiers with per-page risk scoring

Rewrite qa/SKILL.md to v2.0:
- Smart test plan generation with Quick/Standard/Exhaustive tiers
- Per-page risk heuristics (forms=HIGH, CSS=LOW, tests=SKIP)
- Reports persist to ~/.gstack/projects/{slug}/qa-reports/
- QA run index with bidirectional links between reports
- Report metadata: branch, commit, PR, tier
- Auto-open preference saved to ~/.gstack/config.json
- PR comment integration via gh
- file:// link output on completion

qa/templates/qa-report-template.md

@@ -4,12 +4,16 @@
 |-------|-------|
 | **Date** | {DATE} |
 | **URL** | {URL} |
+| **Branch** | {BRANCH} |
+| **Commit** | {COMMIT_SHA} ({COMMIT_DATE}) |
+| **PR** | {PR_NUMBER} ({PR_URL}) or "—" |
+| **Tier** | Quick / Standard / Exhaustive |
 | **Scope** | {SCOPE or "Full app"} |
-| **Mode** | {full / quick / regression} |
 | **Duration** | {DURATION} |
 | **Pages visited** | {COUNT} |
 | **Screenshots** | {COUNT} |
 | **Framework** | {DETECTED or "Unknown"} |
+| **Index** | [All QA runs](./index.md) |
 
 ## Health Score: {SCORE}/100