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).

[diogochaves]

I'd like to take this one if it's still open. After the taskbar work I'd like to get to know more of the project, and documenting the serve endpoints looks like a good way to read through that side of the CLI.

One thing worth flagging: docs/CLI.md has picked up a ## serve section since this was filed, but it only covers the flags — the routes themselves still aren't documented, so I think the issue still stands as written.

[tsouth89]

Yep, still open and it's yours. Good pick after the taskbar work.

Agreed on the docs/CLI.md read — the ## serve section only covers the flags, so I'd just extend that section with the routes rather than start a separate docs/SERVE.md. Keeps it in one place.

One heads up: that section documents --refresh-interval as a cache TTL, but #273 is open because it isn't actually wired up. So don't take the existing prose as ground truth there — go off route_request in rust/src/cli/serve.rs and leave the TTL alone, #273 will fix it separately.