docs(knowledge): document hive-wide knowledge repo + /knowledge bind-mount

The internal/knowledge forge repo and its /knowledge container bind-mount
have been live since feat(#1167) but had no docs/ entry — only a mention
in the agent system prompt.

Add docs/knowledge.md covering:
- Agent access (/knowledge read-only bind-mount inside every container)
- Repository layout (internal/knowledge, auto-seeded README)
- Sync mechanism (webhook on push + periodic pull fallback)
- State paths (/var/lib/hyperhive/knowledge on host)
- Contribution workflow (fork + PR, no direct push)

Also add a file-map entry and reading-path bullet in CLAUDE.md.

CLAUDE.md

@@ -388,6 +388,8 @@ docs/
                        env vars, matrix federation pointer
   ci.md                hive-ci Forgejo Actions runner: options, container
                        shape, auto-registration flow, nix sandbox note
+  knowledge.md         hive-wide knowledge repo: forge clone, /knowledge
+                       bind-mount, webhook + periodic sync, contribution flow
   coordinator.md       rebuild queue: queue kinds (Rebuild, MetaUpdate, Spawn),
                        dedup logic, sources, cascade tracking, step labels;
                        serialisation rationale
@@ -438,6 +440,9 @@ read them à la carte.
   [`docs/coordinator.md`](docs/coordinator.md).
 - **"How does the CI runner work? What's the auto-registration flow?"** →
   [`docs/ci.md`](docs/ci.md).
+- **"What is `/knowledge`? How does the hive-wide knowledge repo sync,
+  and how do I contribute a document?"** →
+  [`docs/knowledge.md`](docs/knowledge.md).
 
 ## Conventions & process