feat: add 6 gap-closing skills — browser QA, design system, product lens, canary watch, benchmark, safety guard

Closes competitive gaps with gstack:
- browser-qa: automated visual testing via browser MCP
- design-system: generate, audit, and detect AI slop in UI
- product-lens: product diagnostic, founder review, feature prioritization
- canary-watch: post-deploy monitoring with alert thresholds
- benchmark: performance baseline and regression detection
- safety-guard: prevent destructive operations in autonomous sessions

skills/product-lens/SKILL.md

@@ -0,0 +1,79 @@
+# Product Lens — Think Before You Build
+
+## When to Use
+
+- Before starting any feature — validate the "why"
+- Weekly product review — are we building the right thing?
+- When stuck choosing between features
+- Before a launch — sanity check the user journey
+- When converting a vague idea into a spec
+
+## How It Works
+
+### Mode 1: Product Diagnostic
+
+Like YC office hours but automated. Asks the hard questions:
+
+```
+1. Who is this for? (specific person, not "developers")
+2. What's the pain? (quantify: how often, how bad, what do they do today?)
+3. Why now? (what changed that makes this possible/necessary?)
+4. What's the 10-star version? (if money/time were unlimited)
+5. What's the MVP? (smallest thing that proves the thesis)
+6. What's the anti-goal? (what are you explicitly NOT building?)
+7. How do you know it's working? (metric, not vibes)
+```
+
+Output: a `PRODUCT-BRIEF.md` with answers, risks, and a go/no-go recommendation.
+
+### Mode 2: Founder Review
+
+Reviews your current project through a founder lens:
+
+```
+1. Read README, CLAUDE.md, package.json, recent commits
+2. Infer: what is this trying to be?
+3. Score: product-market fit signals (0-10)
+   - Usage growth trajectory
+   - Retention indicators (repeat contributors, return users)
+   - Revenue signals (pricing page, billing code, Stripe integration)
+   - Competitive moat (what's hard to copy?)
+4. Identify: the one thing that would 10x this
+5. Flag: things you're building that don't matter
+```
+
+### Mode 3: User Journey Audit
+
+Maps the actual user experience:
+
+```
+1. Clone/install the product as a new user
+2. Document every friction point (confusing steps, errors, missing docs)
+3. Time each step
+4. Compare to competitor onboarding
+5. Score: time-to-value (how long until the user gets their first win?)
+6. Recommend: top 3 fixes for onboarding
+```
+
+### Mode 4: Feature Prioritization
+
+When you have 10 ideas and need to pick 2:
+
+```
+1. List all candidate features
+2. Score each on: impact (1-5) × confidence (1-5) ÷ effort (1-5)
+3. Rank by ICE score
+4. Apply constraints: runway, team size, dependencies
+5. Output: prioritized roadmap with rationale
+```
+
+## Output
+
+All modes output actionable docs, not essays. Every recommendation has a specific next step.
+
+## Integration
+
+Pair with:
+- `/browser-qa` to verify the user journey audit findings
+- `/design-system audit` for visual polish assessment
+- `/canary-watch` for post-launch monitoring