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.

.agents/skills/docs-lint/lint.py

@@ -0,0 +1,69 @@
+#!/usr/bin/env python3
+"""Lint committed docs against .agents/shared/writing-style.md.
+
+Checks two mechanical rules:
+  1. Banned vocabulary (significance puffers, analytical verbs, poetic nouns,
+     promotional adjectives, opening crutches).
+  2. Broken relative markdown links.
+
+Prose-voice rules are not machine-checkable; this covers the parts that are.
+Run from the repo root: python3 .agents/skills/docs-lint/lint.py [paths...]
+Exit 1 if any violation is found.
+"""
+import os, re, sys
+
+BANNED = [
+    "pivotal", "crucial", "vital", "groundbreaking", "transformative", "testament",
+    "paramount", "invaluable", "delve", "leverage", "utilize", "facilitate", "foster",
+    "showcase", "underscore", "streamline", "harness", "tapestry", "realm", "paradigm",
+    "nexus", "cornerstone", "robust", "seamless", "innovative", "cutting-edge",
+    "meticulous", "holistic", "comprehensive", "in today's world",
+    "it's worth noting", "it is important to note",
+]
+BAN_RE = re.compile(r'(?<![\w-])(' + "|".join(re.escape(w) for w in BANNED) + r')(?![\w-])', re.I)
+LINK = re.compile(r'\]\(([^)]+)\)')
+
+def iter_md(paths):
+    for p in paths:
+        if os.path.isfile(p) and p.endswith(".md"):
+            yield p
+        for root, dirs, files in os.walk(p):
+            dirs[:] = [d for d in dirs if d not in (".git", "node_modules")]
+            for f in files:
+                if f.endswith(".md"):
+                    yield os.path.join(root, f)
+
+def main(argv):
+    paths = argv or ["knowledge", ".agents", "operations", "investigations", "plans"]
+    violations = 0
+    # The style guide and this skill enumerate the banned words by definition.
+    ban_exempt = ("shared/writing-style.md", "skills/docs-lint/")
+    for f in sorted(set(iter_md(paths))):
+        check_banned = not any(x in f for x in ban_exempt)
+        fence = False
+        with open(f) as fh:
+            for ln, line in enumerate(fh, 1):
+                if line.lstrip().startswith("```"):
+                    fence = not fence; continue
+                if fence:
+                    continue
+                if check_banned:
+                    for m in BAN_RE.finditer(line):
+                        print(f"{f}:{ln}: banned word '{m.group(1)}'")
+                        violations += 1
+                for m in LINK.finditer(line):
+                    link = m.group(1)
+                    if re.match(r'^(https?:|mailto:|#|/)', link):
+                        continue
+                    path = re.split(r'[#?]', link)[0]
+                    if not path:
+                        continue
+                    tgt = os.path.normpath(os.path.join(os.path.dirname(f), path))
+                    if not os.path.exists(tgt):
+                        print(f"{f}:{ln}: broken link -> {link}")
+                        violations += 1
+    print(f"\n{violations} violation(s)")
+    return 1 if violations else 0
+
+if __name__ == "__main__":
+    sys.exit(main(sys.argv[1:]))