docs: replace both AGENTS.md files with one CLAUDE.md

One agent doc per repo, and it is CLAUDE.md. This repo carried two --
one at the root and one under webber-api/ -- which is the drift problem
in its purest form: two documents, one subject, and no way to know which
the last reader trusted. Written fresh rather than reformatted.

README.md linked to webber-api/AGENTS.md, so that pointer moves with the
file rather than dangling.

The architecture section states the method used to establish what is
live -- import the app inside the container and read sys.modules -- and
then the case where that method fails here. src/domains/tools is absent
from a cold snapshot and is entirely live: each agent's _register_tools
imports its tool package from inside the method body, on every
/agents/run. Absence from a snapshot taken before any request is served
is a timing artifact, not evidence of death, and deleting on that basis
would have removed the tool layer.

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

README.md

@@ -113,7 +113,7 @@ This project uses prefixed tags for independent release cycles:
 
 ## Documentation
 
-- `webber-api/AGENTS.md` - API development guidelines
+- `CLAUDE.md` - Agent development guidelines (repo-wide)
 - `webber-api/docs/COVERAGE.md` - Feature coverage and roadmap
 - `webber-api/docs/architecture.md` - System architecture
 - `webber-cli/README.md` - CLI usage guide