aad5490e74
WebUI sessions construct AIAgent(platform="webui") but PLATFORM_HINTS had no "webui" entry, so the agent received no platform hint at all. The WebUI frontend supports rich MEDIA:/absolute/path previews for images, audio, video, PDF, HTML, CSV, diffs, and Excalidraw, but without a hint the agent either ignores MEDIA: or falls back to Markdown image syntax which silently fails for local files. Add a webui hint that documents the MEDIA: render path and warns against  for local files. Fixes #21883