feat: Slice 1 polish - MiMo V2.5 model, visible reasoning, image attachments, mobile keyboard fix

- Switch conversation agent to xiaomi/mimo-v2.5 (multimodal: text + image)
- Render native reasoning parts as live 'Thinking trace' (streaming open,
  collapsed after completion); inline <think> extraction for streaming models
- Image attachments: picker (up to 4, 10MB each), base64 to Flue
  AgentPromptImage, authenticated blob-URL replay for historical images
- Mobile keyboard viewport fix: visual-viewport hook, fixed shell,
  interactive-widget=resizes-content, header pinned, composer follows keyboard
- Conversation to Signal to proposed Work: Convex persistence, Effect
  validation in @code/work-os, Work cards with exact source provenance
- Streamdown markdown + Mermaid chart rendering in chat messages
- Flue tool turns hidden, reasoning-containing turns remain visible
- Frontend regression tests: keyboard viewport, responsive shell,
  attachment overflow, authenticated images, reasoning traces, transforms
- .env.example updated to xiaomi/mimo-v2.5 config

apps/web/package.json

@@ -15,6 +15,7 @@
     "@code/env": "workspace:*",
     "@code/primitives": "workspace:*",
     "@code/ui": "workspace:*",
+    "@code/work-os": "workspace:*",
     "@flue/react": "1.0.0-beta.9",
     "@flue/sdk": "1.0.0-beta.9",
     "@react-router/fs-routes": "^8.1.0",