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>

knowledge/index.md

@@ -6,7 +6,7 @@ cross-cutting system, synthesized from live state and evidence. Structure and ru
 
 | Section | What it covers |
 |---------|----------------|
-| [wiki/hosts/](wiki/hosts/) | Proxmox host narratives — `hubris`, `strong`. |
+| [wiki/hosts/](wiki/hosts/index.md) | Proxmox host narratives — `hubris`, `strong`. |
 | [wiki/containers/](wiki/containers/index.md) | LXC fleet — one page per container, plus the master table and archaeology. |
 | [wiki/vms/](wiki/vms/index.md) | Virtual machines — ZimaOS, Home Assistant OS. |
 | [wiki/infrastructure/](wiki/infrastructure/index.md) | Cross-cutting systems — DNS, ingress, mesh, storage, auth, monitoring, generated topology. |