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

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