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/module-eval/bao-grants.nix

@@ -1377,6 +1377,19 @@ let
         lib.hasInfix "path \"secret/data/swarm/services/ci-runner/registration-token\" {\n  capabilities = [\"create\", \"update\"]\n}" s
         && !(lib.hasInfix "secret/data/swarm/services/*" s);
     }
+    {
+      # `forge::classifier_account` mints the review classifier's forge token
+      # and reads it back to decide on a re-mint and to post reviews. Pinned
+      # as the whole stanza, and spelled to the leaf, for the reason above.
+      name = "the controller's grant on the review classifier is its forge token alone";
+      ok =
+        let
+          s = baoGrantHere.systemd.services.swarm-bao-controller-policy.script;
+        in
+        lib.hasInfix "path \"secret/data/swarm/services/review-classifier/forge/token\" {\n  capabilities = [\"create\", \"read\", \"update\"]\n}" s
+        && !(lib.hasInfix "secret/data/swarm/services/review-classifier/*" s)
+        && !(lib.hasInfix "secret/data/swarm/services/review-classifier/anthropic-api-key" s);
+    }
     {
       # The runner's role is the token's only reader. Its policy is the one
       # leaf, read-only, under the runner's own subject.