WS-3: Canvas MCP/CLI surface (stdio)

[espetro]

Plan: .agents/plans/2026-07-04-detach-canvas-pipeline.md (WS-3)

Ship packages/canvas as MCP server + CLI for coding agents (pi.dev, Claude Code).

• MCP server (packages/canvas/bin/mcp-server.ts) via @modelcontextprotocol/sdk: addDesignFrame, updateFrame, removeFrame, getCanvas, exportScreenshot
• Transport: stdio default (local agent integration); streamable-http opt-in
• CLI (packages/canvas/bin/canvas.ts): dev, serve, mcp, export
• Skill file at ~/.config/calca/skills/canvas-design.md (agentskills.io format)

Effort: M (~2d). Depends on WS-1. Related #19