Replaces the fork-and-PR instructions with the supported no-fork path
now that the CLI helper shipped: hive-forge clone + pr-create --agit.
Agents are read-only collaborators, so they push HEAD to
refs/for/<base>/<topic> via AGit instead of forking. Same --topic
updates the open PR.
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.
