docs: move the working agreement to AGENTS.md

The guidance in here is not Claude-specific — it is what this project is,
what it refuses to do, and the rules that are invisible from the code. Any
agent working here needs it, and AGENTS.md is the name they all look for.

CLAUDE.md stays as a pointer so Claude Code still finds it, and holds no
guidance of its own; two copies would drift and the wrong one would be read.
Every cross-reference in the source comments, docs, workflow and skills now
names AGENTS.md, and the image ignores both.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

src/ingest/sanitize.ts

@@ -16,7 +16,7 @@ import { decodeEntities } from "./html.ts";
  *     sanitiser's job stops at prose and URLs.
  *  2. **Clean, do not drop.** A hostile title is truncated and stripped, not
  *     rejected — an event vanishing without a trace is the failure mode this
- *     codebase fears most (CLAUDE.md § Silent drops). The one unrecoverable
+ *     codebase fears most (AGENTS.md § Silent drops). The one unrecoverable
  *     case is a title that sanitises to nothing, and that emits a note the
  *     caller is expected to surface.
  *  3. **Never throw on junk.** Malformed entities, lone surrogates, absurd code