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

nix/host-modules/swarm-bao.nix

@@ -437,6 +437,10 @@ let
       capabilities = ["create", "update"]
     }
 
+    path "${credentialMountPath}/data/${classifierForgeTokenLeaf}" {
+      capabilities = ["create", "read", "update"]
+    }
+
     path "${credentialMountPath}/data/${swarmAppserviceTokenLeaf}" {
       capabilities = ["read"]
     }
@@ -567,6 +571,12 @@ let
   # wrote in memory); the CI runner's fetch unit is its one reader.
   ciRunnerTokenLeaf = "swarm/services/ci-runner/registration-token";
 
+  # The review classifier's forge token, the nix half of
+  # `swarm_secret_client::review_classifier::forge_token_path`. The controller
+  # mints it, reads it back before deciding whether to re-mint, and posts the
+  # classifier's reviews with it; no other principal's policy reaches it.
+  classifierForgeTokenLeaf = "swarm/services/review-classifier/forge/token";
+
   # The UI's OIDC client secret. The publisher writes it; the granter reads it
   # into the `oidc` auth method's config, which is the only copy bao uses.
   baoUiClientLeaf = "swarm/services/${cfg.ui.oidc.clientId}/oidc/client";