feat(scene): track automation mesh import bridge

[DrHepa]

Pre-flight Checks

• I searched existing issues and did not find a duplicate for scene mesh import automation.

Problem Description

The CLI/MCP automation flow can generate meshes headlessly, but Desktop needed an explicit bridge-backed way to import a workspace mesh into the visible UI scene without using file picker automation or the Add to Scene node.

Proposed Solution

Track the Desktop/Electron automation import bridge:

• Advertise scene.import_mesh through GET /automation/capabilities.
• Support POST /scene/import-mesh for workspace-relative .glb, .obj, .stl, and .ply meshes.
• Validate paths safely in Desktop/Electron.
• Reuse the backend import/serve path and update the renderer scene through an explicit event.

Current Status

Implemented and pushed for review/testing:

• Branches updated: modly-private and dev
• Commit: 07072f5 feat(scene): add automation mesh import bridge
• E2E smoke: generated mesh can be imported into the UI through the bridge/CLI flow.

Affected Area

Desktop/Electron automation bridge, renderer scene update, CLI/MCP integration.