feat(#3255): register the swarm-wide forge hooks against the controller

The endpoint landed inert: nothing pointed at it, so the only way to see
it work was to mint an HMAC by hand. Register the two swarm-wide hooks
at startup so a real forge event produces a journal line.

Registered ALONGSIDE the per-hive hooks, not instead of them. Every hive
keeps receiving and acting on its own deliveries; the controller gets a
copy and logs it. Moving the registration is a later step and has to be:
fan-out swarm->hive does not exist yet, so a hook moved now would point
at a receiver that forwards nowhere, silently on both sides.

Deliberately no stale-hook deletion arm, unlike the two per-hive
registrars this otherwise mirrors: theirs delete hooks matching their own
path with a foreign base, and the hives' hooks are not stale.

The route prefix is what keeps this safe. Both hive-side registrars
delete any hook ending in /webhook/knowledge or /webhook/config-pr with a
different base, so a swarm hook under those paths would be deleted by
every hive on every boot. Serving them under /webhook/forge/ avoids it,
and a test pins it -- there is nothing else that can.

SWARM_CONTROLLER_PUBLIC_URL is set only where the swarm vhost is served,
because a hook whose target_url nothing answers is worse than no hook.

docs/swarm/README.md

@@ -349,6 +349,30 @@ list, which would look like a silent swarm rather than a controller that
 cannot see. The body says which. Status survives a controller restart:
 it is stored in the queue, not in the daemon.
 
+### Swarm-wide forge webhooks
+
+At startup the controller registers two Forgejo hooks pointing at
+itself — a `push` hook on `internal/knowledge` and a `pull_request` hook
+on the `agent-configs` org, both under
+`https://<swarm.ui.domain>/webhook/forge/`.
+
+**You will see two hooks where there used to be one**, and that is the
+intended state for now: each hive still registers and receives its own,
+and the controller's is an additional copy that it currently logs and
+nothing more. **Do not delete either.** Removing a hive's hook stops that
+hive acting on knowledge pushes and config PRs; removing the
+controller's just gets recreated on its next start.
+
+Nothing to configure. The hooks are registered only when this host also
+serves the swarm UI vhost — that is what publishes the endpoint, and a
+hook the forge cannot reach would collect failed deliveries while
+looking healthy. The HMAC secret is generated on first start and kept
+(see [`docs/persistence.md`](../persistence.md)).
+
+To check it is working, push to `internal/knowledge` and look for
+`webhook: verified delivery` in `journalctl -u swarm-controller`. A
+refused delivery logs `webhook: refused delivery` with the reason.
+
 ## Cross-references
 
 - `docs/snapshot-store.md` — the swarm's `btrfs receive` endpoint, and