Add AGENTS.md with the repository's rules for agents (closes #246)

AGENTS.md holds the branching model, the entrypoints, the editing rule
and two iron rules: query strings are logged as received, and the
trusted-proxy default is the RFC 1918 set. README links it; SPEC.md
step 2 now states that default and that a given list replaces it.

Model: opus-5-5

AGENTS.md

@@ -0,0 +1,39 @@
+# Agents
+
+These are the rules for agents working on this repository.
+
+## Branches
+
+- `main` is the stable branch, and `next` is the development branch.
+- Every PR is cut from `next` and targets `next`.
+- Only sneak merges `next` into `main`.
+- Each issue lands on `next` as one squashed commit whose title ends
+  ` (closes #N)`. A PR of exactly two commits lands as both commits: first tests
+  that fail against the current code, then the fix with no test assertion
+  changed.
+
+## Entrypoints
+
+- Work through the `Makefile` targets, which call the scripts in `script/`. The
+  README's [Entrypoints](README.md#entrypoints) section lists them.
+- Never run `go`, `golangci-lint` or prettier directly.
+- `make check` passes before every push.
+
+## Editing
+
+- Edit files by hand.
+- Format only with `make fmt`, and run it before committing any change.
+
+## Iron rules
+
+- The request log records query strings exactly as received.
+    - Secrets belong in POST data. An app that puts a secret in a query string
+      is fixed in the app.
+    - Never propose redacting a query string, Gitea's `?token=` included.
+- The default of `SWRP_TRUSTED_PROXIES` is the RFC 1918 set.
+    - The set is:
+        - `10.0.0.0/8`;
+        - `172.16.0.0/12`;
+        - `192.168.0.0/16`.
+    - `SPEC.md` states that default.
+    - Forwarded headers count only when the peer is a trusted proxy.

README.md

@@ -746,6 +746,7 @@ on the host.
 - [`EVALUATION.md`](EVALUATION.md): what already exists, what each tool covers
   and misses, and why none was adopted.
 - [`REPO_POLICIES.md`](REPO_POLICIES.md): the policies this repository follows.
+- [`AGENTS.md`](AGENTS.md): the rules for agents working on this repository.
 - [`docs/settings.md`](docs/settings.md): every setting and the values it takes.
 - [`docs/rules.md`](docs/rules.md): the rule file format and the default file.
 - [`docs/admin-endpoints.md`](docs/admin-endpoints.md): the endpoints under

SPEC.md

@@ -133,6 +133,8 @@ request log's `heuristic` field gives that name. `refusal_limit` and
 1. `GET /_sww/healthz` is answered at once by `rproxy`, before any check, so a
    health checker never needs an exemption and is never refused.
 2. The client is identified.
+    - `SWRP_TRUSTED_PROXIES` is `10.0.0.0/8`, `172.16.0.0/12` and
+      `192.168.0.0/16` by default. A list given replaces them.
     - If the TCP peer is inside `SWRP_TRUSTED_PROXIES`, `X-Forwarded-For` is
       walked from the right, and the first address outside those netblocks is
       the client. If every address is inside them, the leftmost is, so that a