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/wiki/hosts/index.md

@@ -0,0 +1,9 @@
+# Hosts
+
+The two Proxmox VE nodes that run the fleet. Narrative pages; the machine-readable host records are
+the generated `hosts/*.yaml` files at the repo root.
+
+| Document | What it covers |
+|----------|----------------|
+| [hubris.md](hubris.md) | Primary PVE node (GMKtec NucBox M6 Ultra, `192.168.8.77`) — runs most LXCs plus the VMs. |
+| [strong.md](strong.md) | Secondary PVE node / cluster member (`192.168.178.181`) — hosts the LXCs migrated from hubris. |