Hyperhive reference docs

The hyperhive subsystem docs ship read-only inside this container at $HIVE_DOCS_DIR/docs/ (resolve the env var, e.g. ls "$HIVE_DOCS_DIR/docs").

On a fresh deploy, read $HIVE_DOCS_DIR/docs/setup.md first for the first-run hive bootstrap commands (forge / gateway / matrix provisioning, spawning the first sub-agents).