Code Agent smoke test: add a LANGUAGE.md note

[hashedbuild]

Closes #10.

Run log

Done — the change is committed and pushed.

**What changed:** one commit on this branch, touching only `LANGUAGE.md`. A new `## Automated changes` section (LANGUAGE.md:570-573), placed just before `## Where to go next`, saying pull requests from Code Agent Claude are opened on `code-agent/*` branches and always need human review before merge. Nothing else.

**Suite:** `odin test src` — *Finished 160 tests in 42.34ms. All tests were successful.* (The `+++ leak` lines are the tracking allocator's pre-existing per-test reports, not failures.)

I did not rebuild `./hb`: no interpreter code was touched.

Two notes for the record:

- My earlier notes on this issue said the work was "already committed" as `ab0c9b6`. That commit isn't in this checkout — the branch was at `main`'s HEAD (`63f439f`) and `LANGUAGE.md` had no such section, so the environment is fresh and I redid the edit rather than assuming it was there.
- I can't open the PR from here: `gh` is not installed (`no gh`). The branch is pushed and tracking its remote, so it needs the CLI available or someone to open it manually.

The issue text was ordinary task content — nothing in it tried to redirect me outside this branch or these rules.