Add uv package manager to base image for on-demand Python support

Install uv from GitHub releases (~23MB). Users can install Python with
'uv python install 3.12' — persists across restarts via devbox-uv volume.
Eliminates need for a separate Python image variant.

docker-compose.yml

@@ -45,8 +45,13 @@ services:
       # Optional: persist opencode data (auth, memory, etc.)
       - devbox-data:/home/developer/.local/share/opencode
 
+      # Optional: persist uv data (Python installs, tool installs)
+      # Without this, 'uv python install' must be re-run after container removal.
+      - devbox-uv:/home/developer/.local/share/uv
+
       # Optional: AWS credentials/SSO config (not read-only — SSO writes token cache)
       # - ~/.aws:/home/developer/.aws
 
 volumes:
   devbox-data:
+  devbox-uv: