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/research/manifest-format-and-grouping.md

@@ -130,7 +130,7 @@ preserves this format.
 **Pros**
 
 - Zero migration cost.
-- Stdlib parser; no new dependency. The project's CLAUDE.md sets
+- Stdlib parser; no new dependency. The project's AGENTS.md sets
   "low dependencies by default" as a guideline.
 - Stable, predictable parse semantics. No type-coercion gotchas.
 - Tooling everywhere — IDE support, linters, jq.
@@ -162,7 +162,7 @@ ruamel.yaml).
 
 - **New runtime dependency.** The project today uses zero
   third-party Python packages for production code; YAML parsing
-  pulls in PyYAML. (CLAUDE.md: "Python, stdlib-first; low-deps by default.")
+  pulls in PyYAML. (AGENTS.md: "Python, stdlib-first; low-deps by default.")
 - YAML's footguns: indentation sensitivity, the Norway problem
   (`country: NO` → boolean False), implicit type coercion that's
   surprised non-trivial production projects.