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-secret-client/src/review_classifier.rs

@@ -1,9 +1,12 @@
-//! The review classifier agreement: where its inference key and its queue
-//! client secret live in the store, and what each object holds.
+//! The review classifier agreement: where its inference key, its queue
+//! client secret and its forge token live in the store, and what each object
+//! holds.
 //!
 //! The operator writes the key by hand in the store's UI; the classifier
-//! reads both under its own certificate, the key once per run. Its read
-//! stanza in `swarm-bao.nix` names exactly these two paths.
+//! reads the key and the client secret under its own certificate, the key
+//! once per run. Its read stanza in `swarm-bao.nix` names exactly these two
+//! paths. The forge token is `swarm-controller`'s alone: it mints it, and
+//! posts the classifier's reviews with it.
 
 use serde::{Deserialize, Serialize};
 
@@ -35,6 +38,17 @@ pub struct ApiKey {
     pub api_key: String,
 }
 
+/// The path holding the token of the classifier's forge account, as a
+/// [`crate::forge::Credential`].
+///
+/// # Errors
+/// Never in practice: the name segment is a constant. The `Result` is
+/// [`principal_prefix`]'s.
+pub fn forge_token_path() -> Result<String, Error> {
+    let prefix = principal_prefix(Kind::Service, SERVICE)?;
+    Ok(format!("{prefix}/forge/token"))
+}
+
 /// The path holding the queue client secret of the classifier's OIDC client
 /// `client_id`, where `swarm-secret-publisher.nix` puts every swarm service's.
 ///
@@ -61,11 +75,15 @@ mod tests {
     #[test]
     fn the_paths_are_the_ones_the_store_grants() {
         // Spelled out: the operator types the first into the store's UI, and
-        // `swarm-bao.nix` grants both literals.
+        // `swarm-bao.nix` grants each literal.
         assert_eq!(
             api_key_path().expect("a constant segment is legal"),
             "swarm/services/review-classifier/anthropic-api-key"
         );
+        assert_eq!(
+            forge_token_path().expect("a constant segment is legal"),
+            "swarm/services/review-classifier/forge/token"
+        );
         assert_eq!(
             client_secret_path("swarm-review-classifier").expect("a plain id is legal"),
             "swarm/services/swarm-review-classifier/oidc/client"