feat(brand): add local brand images for HA brands proxy API

Added brand/ directory to custom_components/tibber_prices/ with all
8 supported PNG variants, generated from existing SVGs in images/:

- icon.png / dark_icon.png (256×256)
- icon@2x.png / dark_icon@2x.png (512×512)
- logo.png / dark_logo.png (500×128)
- logo@2x.png / dark_logo@2x.png (1000×256)

Local brand images automatically take priority over CDN images and
are served via the HA brands proxy API (/api/brands/integration/).
Silently ignored on HA < 2026.3, no changes to manifest.json needed.

Updated AGENTS.md to document the brand/ directory under "ALLOWED in root".

Impact: Integration icon and logo now display correctly in HA ≥ 2026.3
without requiring a separate submission to the HA brands repository.

AGENTS.md

@@ -313,6 +313,7 @@ After successful refactoring:
 -   Platform modules: `__init__.py`, `sensor.py` (deprecated, now `sensor/`), `binary_sensor.py` (deprecated, now `binary_sensor/`), future platforms
 -   Core integration files: `const.py`, `manifest.json`, `services.yaml`, `diagnostics.py`, `data.py`
 -   Translation directories: `translations/`, `custom_translations/`
+-   Brand images: `brand/` (icon.png, dark_icon.png, logo.png, dark_logo.png + `@2x` variants) — served via HA brands proxy API (HA ≥ 2026.3), silently ignored on older versions
 
 **❌ PROHIBITED in root:**
 -   Utility modules (use `/utils/` package instead)