docs: rename CLAUDE.md to AGENTS.md and rebrand provider-agnostic

Delete CLAUDE.md in favor of AGENTS.md as the orientation doc, rebrand
the project from Codex-bottle to provider-agnostic bot-bottle, and
repoint every CLAUDE.md reference across PRDs, research notes, the
implementer agent example, and the yaml_subset comment.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/prds/0010-cred-proxy.md

@@ -415,7 +415,7 @@ cannot smuggle a stolen token through this path.
 The proxy binary. Two real options:
 
 - **Python (stdlib)** — `http.server` + `urllib`/`http.client`,
-  no new pip packages. Matches CLAUDE.md's "Python, stdlib-first,
+  no new pip packages. Matches AGENTS.md's "Python, stdlib-first,
   low-deps" posture. SSE pass-through is fiddly but doable.
 - **Go single binary** — cleaner SSE story, smaller runtime,
   one static binary in a scratch/distroless image. New build