docs: add AGENTS.md with documentation-consistency rule

Docs must never contradict the code: verify claims against the codebase,
update docs in the same commit as public-surface changes, keep example
commands copy-paste runnable, verify commands empirically before
committing.

AGENTS.md

@@ -0,0 +1,18 @@
+# Agent Instructions
+
+## Documentation consistency (hard rule)
+
+Documentation must never contradict the code:
+
+- **Check before you write.** Before changing or adding any doc, verify every claim
+  (commands, binaries, tool names, env vars, parameters, tool counts) against the
+  actual state of `package.json`, `src/` and the build output. No stale instructions.
+- **Code and docs change in the same commit.** Every change that alters the public
+  surface — tools added/removed, env vars, CLI flags, binary names, npm scripts,
+  dependency majors — must update `README.md`, `CLAUDE.md` (tool list) and
+  `env.example` in the same change.
+- **Commands must be copy-paste runnable.** Example commands in docs must work as
+  written (correct binary names, required env vars shown). Prefer documenting the
+  happy path plus the exact failure mode when a missing env var is required.
+- **When in doubt, verify empirically.** Run the command in the docs before
+  committing it (`--version` smoke, or a dry call), the same way code is verified.