The brain HTTP REST /query endpoint accepts POST with JSON
{query, limit}, not GET with URL query string. Surfaced by
Task 7 smoke testing — GET returned 405 Method Not Allowed.
The response shape ({results:[...]}) is unchanged; only the
request side flips to POST + JSON body. brainClient.Write was
already using POST + JSON body and is unaffected.
Tests updated to assert POST + JSON body on the Query path.
Adds brainClient with Query and Write methods against the brain's
HTTP REST endpoints (/query, /write). Constructor reads BRAIN_URL env
var, defaulting to http://koala:30330 — the Tailscale-exposed
NodePort that serves both MCP and REST.
Tests cover success, transport error, and non-200 cases via
httptest fakes; URL override is verified via t.Setenv.
