AGENTS.md: an overview for agents; car-emulator in the flake

AGENTS.md still described upstream's EVABRPUploader (GitHub updates,
the old architecture and tabs). It is now the entry point for agents:
what the project is (app, Go server, k8s, EVHardware), the rules
(nothing touches the cluster, secrets, read-only telemetry, null is not
zero, fail closed, honesty), the facts confirmed on the car, the app's
structure and UI conventions, how to build, test, publish and install,
and which document holds what. SPECS.md stays the journal.

The Android 9 test emulator, so far a file under the ignored build/,
is now reproducible: nix run .#car-emulator creates an AVD at
1920x720/160 dpi with the AOSP image that shares the car's platform key
and starts it on port 5560. Checked from scratch: the car build
installs as the system user.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QLJfZQZeAKAwSqViTmLzki

SPECS.md

@@ -112,6 +112,10 @@ tyre pressures, GPS (lat, lon, elevation, heading) and the is_* flags. It does *
   INGEST_TOKEN) into git and pushed it. Commit amended and force-pushed; both values regenerated in
   the now gitignored `k8s/secrets.yaml` (since moved to `k8s/secrets/secrets.yaml`, outside
   `kubectl apply -f k8s/`). Filled-in secrets live only there from now on.
+- 2026-10-18 — AGENTS.md rewritten as the entry point for agents: what the project is, rules,
+  confirmed car facts, app structure, UI conventions, build/test/ship, documents. It replaces
+  upstream's EVABRPUploader text. The test emulator is now in the flake: `nix run .#car-emulator`
+  (Android 9 AOSP at 1920×720, 160 dpi; the car build installs as the system user — checked).
 - 2026-10-18 — App 2.18.0: Settings → App → "Install from USB stick" (car build only): the newest
   `NilsMG4-unstable-<version>.apk` newer than the installed one in a stick's top folder, checked
   against its `.sha256` and the package, installed with the OTA installer session. Tested on the