TASK-IMP-059: Wiki link-integrity gate

Summary

Add a docs gate that fails on broken relative markdown links and missing TASK- cross-refs in frontmatter, so the corpus agents plan from cannot rot silently (deep-audit R51).

Problem

Task READMEs and specs accumulate dead relative links and depends_on edges pointing at tasks that never existed or were renumbered. Agents treat those paths as real.

Proposed Solution

Ship tools/install/docs-tools/wiki-link-check.mjs (+ allowlist), wire it into the install test suite and payload vendor copy, and keep the live docs/ scan green via allowlist for known historical index debt.

Alternatives Considered

• Only warn (non-zero never) — rejected: green theatre.
• Fail on every bare prose TASK- mention — rejected: specs cite future work; scope is links + frontmatter edges.

Success Metrics

• Suite test_wiki_link_check.sh pass=6 fail=0.
• Live docs/ scan exits 0 with committed allowlist.

Scope

In scope: checker, allowlist, test suite, build.sh vendor lines.

Out of scope / Non-Goals

• Rewriting every historical module README index (allowlisted; follow-up sweep).
• HTML docs-site link checking.

AI Authorship Disclosure

• Tools used: Composer session agent.
• Human review: session HITL with Stephen Cheng (session operator).

1. Description (normative)

• 1.1 MUST provide wiki-link-check.mjs that scans docs/**/*.md (excluding _archive/), fails on broken relative non-http file links, skips .html targets, and supports an allowlist file.
• 1.2 MUST fail when depends_on / blocks / related_tasks frontmatter on spec.md cites a TASK-XXX id with no matching docs/tasks/**/TASK-XXX* folder, unless allowlisted.
• 1.3 MUST ship tools/install/tests/test_wiki_link_check.sh covering help, pass/fail fixtures, allowlist, live scan, and build vendor grep.
• 1.4 MUST vendor the checker (+ allowlist) from tools/install/build.sh.

2. Acceptance criteria

• [x] AC 1 (traces_to: #1.1, #1.2) - checker exits 1 on broken link / missing TASK; 0 when clean
• [x] AC 2 (traces_to: #1.3) - test_wiki_link_check.sh pass=6 fail=0
• [x] AC 3 (traces_to: #1.4) - build.sh copies wiki-link-check.mjs and allowlist

3. Edge cases

• Pure #anchor links are not file targets.
• Allowlist keys: file -> target, bare TASK-ID, or whole-file path.