add review classifier rules for this repo

Eight classifications in .hyperhive/review-classifier.yml: no AI
advertising in commit messages, no committed secrets, no clippy
allow attributes, no debug leftovers, no dead code, tests with
behaviour changes, docs with operator-facing changes, one purpose per PR.

A swarm-controller test parses the real file so it can't rot, and the
rust source fileset in nix/rust.nix includes it so the test finds it
in the build sandbox.

.hyperhive/review-classifier.yml

@@ -0,0 +1,52 @@
+classifications:
+  - name: no-ads
+    description: >-
+      A commit message contains a co-authored-by line or other advertising
+      for an AI model, such as a "generated with" footer.
+    polarity: negative
+    threshold: 80
+  - name: no-secrets
+    description: >-
+      The diff adds a secret as literal text: a password, API key, access
+      token or private key. A path or name that points at a secret is not
+      one.
+    polarity: negative
+    threshold: 60
+  - name: no-lint-suppression
+    description: >-
+      The diff adds a Rust `#[allow(clippy::...)]` or `#![allow(clippy::...)]`
+      attribute instead of fixing what the lint reports.
+    polarity: negative
+    threshold: 70
+  - name: no-debug-leftovers
+    description: >-
+      The diff adds leftover debugging: a `dbg!` call, a print statement
+      that only inspects a value, or a block of commented-out code.
+    polarity: negative
+    threshold: 70
+  - name: no-dead-code
+    description: >-
+      The diff adds a function, type, option or module that nothing in the
+      diff calls, uses or registers, and that is not a test, a trait
+      implementation or public API.
+    polarity: negative
+    threshold: 75
+  - name: tests-with-behaviour
+    description: >-
+      Every change to how the code behaves comes with a new or updated test
+      that exercises it.
+    polarity: positive
+    threshold: 65
+  - name: docs-updated
+    description: >-
+      A change to what an operator or user sees, such as a command, flag,
+      option, endpoint, service or default, updates the matching page under
+      docs/ or the crate's README in the same diff.
+    polarity: positive
+    threshold: 65
+  - name: one-purpose
+    description: >-
+      Every change in the diff serves what the commit messages say the PR
+      does, and nothing unrelated is bundled in.
+    polarity: positive
+    threshold: 65