docs: section-index pass + docs-lint skill (phase 5)

- Add knowledge/wiki/hosts/index.md (the one missing section index) and point
  knowledge/index.md at it.
- Add .agents/skills/docs-lint/ (SKILL.md + lint.py) enforcing the mechanical
  parts of writing-style.md: banned vocabulary and broken relative links. The
  style guide and this skill are exempt from the banned-word check since they
  enumerate the list.
- Record the restructure + lint in knowledge/log.md.

Verification: banned-vocabulary scan of knowledge/ is clean (the few remaining
repo-wide hits are false positives — the literal '_' character — or historical
append-only plans quoting the vocabulary, which the standard does not restyle).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.agents/skills/docs-lint/SKILL.md

@@ -0,0 +1,25 @@
+---
+name: docs-lint
+risk_class: read_only
+inputs: [paths]
+verification: "python3 .agents/skills/docs-lint/lint.py"
+docs_update_checklist: []
+---
+
+# Docs lint
+
+Check committed documentation against the mechanical rules in
+[writing-style.md](../../shared/writing-style.md): banned vocabulary and broken relative markdown
+links. Prose-voice rules are not machine-checkable — those stay a review responsibility.
+
+Run from the repo root:
+
+    python3 .agents/skills/docs-lint/lint.py            # default: knowledge/ .agents/ operations/ investigations/ plans/
+    python3 .agents/skills/docs-lint/lint.py knowledge/wiki/containers/104-gitea.md
+
+Exit code is non-zero when any violation is found, so it can gate a commit. The banned-vocabulary
+list mirrors `writing-style.md`; update both together if the standard changes.
+
+> **Known baseline.** The lab carries pre-existing broken links to destroyed/archived nodes (e.g.
+> `124-authentik.md`, now `106-auth-outpost`). Clean those opportunistically; do not treat the
+> current count as a regression from this skill.