Document the local codexbar serve HTTP endpoints

[tsouth89]

Context

codexbar serve runs a small local HTTP server (127.0.0.1) that returns usage/cost JSON — handy for scripting — but the routes are not documented for users. From rust/src/cli/serve.rs the routes are:

• GET /health
• GET /usage (optional ?provider=<id>)
• GET /cost (optional ?provider=<id>)

Task

Document these endpoints — in a ## serve section of docs/CLI.md (if that doc exists by then) or a short docs/SERVE.md. Include the default host/port, each route, the provider query param, and a curl example with a sample JSON shape.

Acceptance criteria

• All three routes documented, plus the 404 / bad request behavior.
• At least one runnable curl example.
• Notes that the server is local-only (127.0.0.1) and read-only.

How to test

Docs only. To confirm routes, start the server and curl it:

cargo run --manifest-path rust/Cargo.toml -- serve
# then, in another shell:
curl http://127.0.0.1:8080/health

Pointers

• Routing: rust/src/cli/serve.rs:57 (route_request).