feat: documentation quality rubric + validate-docs companion reference

[dacervera]

What

• New builtin rubric: documentation.json (12 criteria across 6 categories: claim_discipline, consistency, boundary, accuracy, navigation, quality)
• Bidirectional cross-reference between validate-docs.py and the documentation rubric via companion_tools metadata
• Mechanical checks (validate-docs.py) complement semantic evaluation (rubric)

Why

PR #16 validation revealed no documentation rubric existed. The research-synthesis rubric produced >50% NOT_APPLICABLE on markdown docs. This rubric was designed specifically for project documentation: READMEs, specs, changelogs, tutorials, and guides.

Files Changed

• reference-implementation/quorum/rubrics/builtin/documentation.json (new)
• tools/validate-docs.py (companion docstring added)

Criteria Summary

Category	IDs	Severities
claim_discipline	DOC-001, 002, 004, 012	1 CRITICAL, 2 HIGH, 1 MEDIUM
consistency	DOC-003, 005	2 HIGH
boundary	DOC-006	1 CRITICAL
accuracy	DOC-007, 011	2 HIGH
navigation	DOC-008, 009	2 MEDIUM
quality	DOC-010	1 LOW

DOC-005 and DOC-008 are marked deterministic: true for Tester L1 (zero API cost).

Validation

• validate-docs.py: clean ✅
• Boundary scan: clean (only pre-existing quorum-runs paths) ✅

[dacervera]

CI Triage — Quorum Self-Validation

Verdict: REVISE (10 findings on validate-docs.py)

All findings are pre-existing — this PR only adds 7 lines of docstring text. The file itself was not functionally changed.

2 HIGH — False Positives

Both flag read_file_lines() returning None without caller guards:

• PC-002 (completeness): read_file_lines returns None but callers don't check
• PC-004 (correctness): type safety issue with None return

Both call sites (lines 359 and 411) already have if lines is None: continue — the critics flagged the signature without tracing through to the guards. Verified: no unguarded call sites exist.

6 MEDIUM + 2 LOW — Pre-existing Design Debt

PC-005, PC-006, PC-007, PC-008, PC-025 (SRP, god function, duplication, hardcoded deps, mixed I/O), PC-012 (input validation), PC-017 (magic numbers), PC-009 (resource lifecycle). All pre-existing, same findings surfaced on PR #14 CI run. Routed to stale content backlog.