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

README.md

@@ -748,6 +748,7 @@ 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