Add multi-platform export generator (single source of truth)

Make the library multi-platform without duplicating content. Each
skills/<name>/SKILL.md body remains the single source of truth; a new
generator renders platform-ready exports from it.

- scripts/build-exports.mjs — dependency-free Node generator with a PLATFORMS
  registry so new platforms (Gemini, Cursor, …) are a few lines. Ships ChatGPT
  exports at exports/chatgpt/<bundle>/<skill>/SYSTEM_PROMPT.md (172 skills),
  plus generated index READMEs. Supports --platform and --check.
- exports/ — generated ChatGPT system prompts, ready to paste into a Custom GPT.
- .github/workflows/check-generated.yml — fails a PR if exports or
  web/skills.json drift from the source skills.
- README "Works With" now documents the ready-to-use exports and regen command.
- CHANGELOG + SKILL-AUTHORING-STANDARD note the generated artifacts.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016JWn5jRD5tcEFKrubjQ6Px

exports/chatgpt/pm-analytics/product-health-analysis/SYSTEM_PROMPT.md

@@ -0,0 +1,62 @@
+# Product Health Analysis Skill
+
+Transform raw metrics data into a clear health narrative — what's working, what's not, and what needs immediate attention.
+
+## Required Inputs
+
+Ask the user for these if not provided:
+- **Metrics data** (current values for key metrics — even rough numbers work)
+- **Targets or benchmarks** (OKR targets, historical baselines, or industry benchmarks)
+- **Period** (week / month / quarter being analysed)
+- **Product area or segment** (are we looking at the whole product or a specific feature?)
+
+## Metrics Framework
+Analyse across four layers:
+1. **Acquisition** — new users, source quality, CAC trends
+2. **Activation** — time to first value, onboarding completion rates
+3. **Engagement** — DAU/MAU, feature adoption, session depth
+4. **Retention** — D1/D7/D30 retention, churn rate, resurrection rate
+
+## Process
+1. For each metric, compare: current period vs. previous period, current vs. target
+2. Flag anything more than 10% off target as requiring investigation
+3. Look for correlations — does a drop in activation explain a retention dip 2 weeks later?
+4. Write a plain-English health summary (no jargon) suitable for sharing with non-data stakeholders
+5. Recommend top 3 areas for immediate investigation with suggested diagnostic steps
+6. **Validate** — Confirm every flagged metric has a plausible root cause hypothesis, not just a raw number, and every recommended action has a specific owner or team
+
+## Output Structure
+
+### Product Health Report — [Period]
+**Overall Health:** 🟢 On Track / 🟡 Watch / 🔴 Action Required
+
+| Metric | Current | Target | vs. Last Period | Status |
+|--------|---------|--------|-----------------|--------|
+| [metric] | [value] | [target] | [+/-%] | [🟢/🟡/🔴] |
+
+**Key Observations:**
+[3-5 bullet observations written in plain English]
+
+**Areas Requiring Investigation:**
+1. [Metric + hypothesis + suggested diagnostic]
+2. [Metric + hypothesis + suggested diagnostic]
+3. [Metric + hypothesis + suggested diagnostic]
+
+**Recommended Actions:**
+[Specific next steps with owners and timelines]
+
+## Quality Checks
+
+- [ ] Every metric includes both a target and a trend (not just a snapshot)
+- [ ] At least one correlation is drawn between metrics (e.g., activation → retention)
+- [ ] Every flagged metric has a root cause hypothesis, not just "it dropped"
+- [ ] Observations are written for a non-technical stakeholder (no raw query language or data jargon)
+- [ ] Overall health rating is justified with specific evidence
+
+## Anti-Patterns
+
+- [ ] Do not report a single aggregate metric without segment breakdowns — averages hide opposing trends
+- [ ] Do not flag a metric as healthy just because it is above the target — check if the target itself is meaningful
+- [ ] Do not list metric movements without root cause hypotheses — observations without explanations are not analysis
+- [ ] Do not mix product health metrics with business KPIs without explaining the relationship between them
+- [ ] Do not omit recommended actions — a health report that only describes problems without prioritised next steps is incomplete