Add 5 metric and documentation health docs

docs/evidence/DOCS_HEALTH_METRIC_DEFINITIONS.md

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+# Documentation Health Metric Definitions
+
+This document defines the core metrics used to assess documentation health.
+
+## Metric Definitions
+
+| Metric | Definition | Collection Method | Target |
+|--------|------------|-------------------|--------|
+| Link health | % of internal doc links that resolve | `check_doc_links.py` | 100% |
+| Claim safety | Number of overclaiming phrases | `check_claims.py` | 0 |
+| Index coverage | % of docs linked from PROJECT_INDEX.md | Manual audit | 100% |
+| Freshness | Age since last significant update | `git log` | < 6 months |
+| Onboarding time | Time from clone to demo | Stopwatch | < 15 min |
+| Concept card coverage | % of artifact types with cards | Manual audit | 100% |
+
+## Related
+
+- `docs/evidence/DOCS_READINESS_CHECKLIST.md` — per-release checklist
+- `docs/evidence/DOCS_REVIEW_CADENCE_POLICY.md` — review schedule

docs/evidence/METRIC_GUIDE_CONTRIBUTOR_DOCS.md

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+# Metric Guide: Contributor Docs
+
+**Question:** Does a new contributor have everything they need to make a first change?
+
+**What to track:** Whether all contributor-facing docs cover the full path from setup to merged PR.
+
+**Documents to check:**
+- CONTRIBUTING.md
+- QUICKSTART.md
+- CONTRIBUTOR_CURRICULUM.md
+- RUNBOOK_PICKING_UP_ISSUE.md
+- RUNBOOK_ASKING_FOR_REVIEW.md
+
+**Target:** A contributor can follow the documented path without gaps.
+
+**What good looks like:** A first-time contributor can clone, run the demo, pick an issue, make a change, and submit a PR using only the documented steps.

docs/evidence/METRIC_GUIDE_MAINTENANCE_HEALTH.md

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+# Metric Guide: Maintenance Health
+
+**Question:** Are the docs staying current, or are they accumulating stale content?
+
+**What to track:** Age of last significant update per document.
+
+**Target:** No document older than 6 months without review.
+
+**How to measure:** Check `git log --oneline docs/` for each document.
+
+**What good looks like:** Active documents are updated within the last 3 months. Stale documents are flagged for review or retirement.

docs/evidence/METRIC_GUIDE_NAVIGATION_HEALTH.md

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+# Metric Guide: Navigation Health
+
+**Question:** Can a reader find the document they need without searching?
+
+**What to track:** Whether all docs are linked from PROJECT_INDEX.md.
+
+**Target:** 100% of documents in docs/ are indexed in PROJECT_INDEX.md.
+
+**How to measure:** Compare `find docs/ -name "*.md"` against links in PROJECT_INDEX.md.
+
+**What good looks like:** A new reader can navigate from PROJECT_INDEX.md to any document in one or two clicks.

docs/evidence/METRIC_GUIDE_RELEASE_READINESS.md

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+# Metric Guide: Release Readiness
+
+**Question:** Is the project ready for a public release?
+
+**What to track:** Whether all release artifacts exist and pass checks.
+
+**Artifacts to check:**
+- `make full-reproducibility-report` succeeds
+- `python scripts/check_doc_links.py` reports 0 broken links
+- `python scripts/safety/check_claims.py` — no overclaiming language
+- `docs/review/PUBLICATION_PACK.md` checklist is complete
+
+**Target:** All checks pass.
+
+**What good looks like:** A maintainer can run `make full-reproducibility-report`, review the output, and decide whether to proceed.

tests/test_metric_docs_batch.py

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+"""Tests for metric docs batch."""
+from pathlib import Path
+
+FILES = [
+    "METRIC_GUIDE_RELEASE_READINESS.md",
+    "METRIC_GUIDE_NAVIGATION_HEALTH.md",
+    "METRIC_GUIDE_MAINTENANCE_HEALTH.md",
+    "METRIC_GUIDE_CONTRIBUTOR_DOCS.md",
+    "DOCS_HEALTH_METRIC_DEFINITIONS.md",
+]
+
+
+def test_all_exist():
+    for f in FILES:
+        assert Path(f"docs/evidence/{f}").exists(), f"Missing: {f}"
+
+
+def test_each_has_content():
+    for f in FILES:
+        text = Path(f"docs/evidence/{f}").read_text()
+        assert len(text) > 100, f"{f} too short"