chore: add Cursor Cloud dev setup notes and ignore backend venv

- Add AGENTS.md documenting the card + integration + demo harness,
  and non-obvious cloud caveats (Python 3.13 backend venv for HA-harness
  tests, fresh-bundle copy before smoke, demo render nudge, known
  pixel-precision smoke).
- Ignore .venv-backend/ (Python 3.13 venv provisioned by the update script).

Co-authored-by: Matysh <Matysh@users.noreply.github.com>

.gitignore

@@ -4,3 +4,4 @@ test-build/
 *.log
 __pycache__/
 .pytest_cache/
+.venv-backend/

AGENTS.md

@@ -0,0 +1,19 @@
+# AGENTS.md
+
+House Plan is one HACS package with two parts plus a demo harness:
+
+- **Lovelace card** (`src/`, TypeScript + Lit) — the primary product, bundled to `dist/houseplan-card.js`.
+- **Storage integration** (`custom_components/houseplan/`, Python) — the Home Assistant backend.
+- **Demo harness** (`demo/`) — a self-contained Playwright page (`demo/srv/demo.html`) that renders the card against a fake `hass`, used for screenshots and the `smoke_*.mjs` end-to-end suite.
+
+Standard commands live in `package.json` scripts, `CONTRIBUTING.md`, and `docs/DEVELOPMENT.md`. Read `docs/ARCHITECTURE.md` and `docs/STATUS.md` before non-trivial changes.
+
+## Cursor Cloud specific instructions
+
+The startup update script already runs `npm ci`, provisions a Python 3.13 backend venv at `.venv-backend`, and installs Playwright Chromium. You do not need to reinstall dependencies.
+
+- **Frontend** (from repo root): `npm run typecheck`, `npm test` (node:test, ~270 tests), `npm run build`. After building, keep the integration copy in sync — `cp dist/houseplan-card.js custom_components/houseplan/frontend/`. CI enforces `cmp dist/houseplan-card.js custom_components/houseplan/frontend/houseplan-card.js` byte-for-byte.
+- **Backend HA-harness tests need Python 3.13, not the system 3.12.** Run them with the venv: `.venv-backend/bin/python -m pytest tests_backend/ -q` (126 tests). Running `python3 -m pytest tests_backend` on the system 3.12 silently **skips** the `test_ha_*.py` harness tests (`conftest.py` ignores them when `homeassistant` is not importable) and runs only the ~83 pure tests.
+- **Running the app / smoke suite**: build a fresh bundle and copy it into the demo assets first — `npm run build && cp dist/houseplan-card.js demo/srv/assets/houseplan-card.js` — then run `node demo/smoke_*.mjs`. The committed `demo/srv/assets/houseplan-card.js` is a stale snapshot; testing it reports green about code that no longer exists. No real Home Assistant server is required: `demo/srv/demo.html` stubs `hass`, registries and `callService`.
+- **Demo harness render quirk**: the fake `hass` in `demo.html` is set once, so opening the page directly in a browser renders the floor plan but **device icons only appear after a re-render** (an F5 refresh, or nudging `card.hass = {...card.hass}`). The smoke launcher `demo/serve.mjs` already does this nudge; a plain browser session does not. This is a harness limitation, not a card bug.
+- **Known environment-sensitive smoke**: `demo/smoke_opening_measure.mjs` fails two sub-checks (`place_dialog_x_magnetised`, `place_committed_x_center`) under the pinned Chromium — a `1e-6`-tolerance magnet-snap on the opening-*placement* path. It reproduces against the pristine committed bundle, so treat it as pre-existing/pixel-precision, not a regression you introduced.