swarm-controller: post the review classifier's verdict on a review request

Requesting the forge account `review-classifier` as a reviewer now runs the
repo's rules against the PR and posts the result as that account's review,
on the head commit the run read. A push starts no run; only a fresh review
request does.

- An instance-wide `pull_request_review_request` system hook delivers to
  `/webhook/forge/review-request`. A request for any other reviewer, or a
  removed request, does nothing.
- The controller reads the PR, loads the rules with the same reader as
  `GET /api/review-classifier/rules`, sends one request on the try-out
  subject, and posts APPROVED or REQUEST_CHANGES with the table as the body.
- With no clean verdict (PR or rules don't load, the run fails, or any
  classification has no answer) it posts a COMMENT saying why, never an
  approval. The event is one function, `no_clean_verdict_event`.
- Table cells are flattened, markup-escaped, and every `@` gets a
  zero-width space, so a model's reason can't break the table or mention
  anyone.
- At most two requested reviews run at once; the rest wait in memory.
- The account is minted the way hive accounts are: a five-minute pass
  queues a `MintClassifierForgeToken` node when the token at
  `swarm/services/review-classifier/forge/token` is missing or stale. The
  token is `write:repository`. The controller's bao policy gains that leaf
  (create/read/update), pinned in module-eval bao-grants. The classifier
  never sees the token.

The account gets no repo access automatically; a repo admin adds it by
hand. A re-request retires the account's earlier review: Forgejo clears
`official` on a reviewer's earlier reviews when it is requested again
(models/issues/review.go AddReviewRequest, v16.0.5), and approvals and
rejection blocks count official reviews only.

Refs #5137

swarm-controller/src/webhook.rs

@@ -221,6 +221,10 @@ pub(super) enum DeliveryKind {
     /// `repo_create_hook`) — different scope, same event name.
     #[strum(serialize = "vcs-activity")]
     VcsActivity,
+    /// `pull_request_review_request` events on every repo in the instance,
+    /// for the review classifier's account (`crate::review_classifier::review`).
+    #[strum(serialize = "review-request")]
+    ReviewRequest,
 }
 
 /// The route prefix a registered `target_url` must point at.
@@ -236,7 +240,12 @@ impl DeliveryKind {
     /// Every kind, for the registration sweep. An array rather than a
     /// hand-written list at the call site, so adding a kind cannot leave one
     /// hook unregistered.
-    pub(super) const ALL: [Self; 3] = [Self::Knowledge, Self::ConfigPr, Self::VcsActivity];
+    pub(super) const ALL: [Self; 4] = [
+        Self::Knowledge,
+        Self::ConfigPr,
+        Self::VcsActivity,
+        Self::ReviewRequest,
+    ];
 
     /// The `target_url` to register with Forgejo for this kind, given the
     /// swarm's public base URL.
@@ -341,7 +350,9 @@ pub(super) fn verify(
 /// [`announce_knowledge_change`]); a `ConfigPr` delivery updates
 /// `crate::config_pr::ConfigPrCache` immediately (see that module's doc
 /// comment for why both this and a periodic poll write the same cache), and a
-/// merge queues a deploy of the merged commit on the agent's hive.
+/// merge queues a deploy of the merged commit on the agent's hive. A
+/// `ReviewRequest` delivery naming the review classifier's account starts a
+/// review run in the background.
 ///
 /// Returns 200 on an accepted delivery so Forgejo does not retry. A refused
 /// one answers 401 (bad signature) or 503 (this daemon has no secret), and
@@ -351,7 +362,8 @@ pub(super) fn verify(
     path = "/webhook/forge/{kind}",
     params(
         ("kind" = String, Path, description = "which swarm-wide hook this delivery arrived on: \
-                                                `knowledge` or `config-pr`")
+                                                `knowledge`, `config-pr`, `vcs-activity` or \
+                                                `review-request`")
     ),
     request_body(
         content = String,
@@ -414,6 +426,7 @@ pub(super) async fn post_webhook_forge(
                 tracing::warn!("webhook: vcs-activity delivery did not parse as a push payload");
             }
         }
+        DeliveryKind::ReviewRequest => crate::review_classifier::review::on_delivery(&state, &body),
     }
 
     (StatusCode::OK, "ok").into_response()