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/shared/daily.ts

@@ -25,7 +25,7 @@ import { DAY, HOUR, REGION_RESET_UTC_OFFSET } from "./time.ts";
 export const RESET_HOUR_LOCAL = 4;
 
 /**
- * A 180-day event is already a parse error (see CLAUDE.md § Domain rules), so
+ * A 180-day event is already a parse error (see AGENTS.md § Domain rules), so
  * this only ever fires on data that is wrong; it exists so a bad end date
  * cannot make the client allocate an unbounded array.
  */