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 may instead be exactly two commits: first tests that fail
+  against the current code, then the fix with no test assertion changed. Such a
+  PR lands as both commits.
+
+## 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.