WS-2: Extract pipeline as packages/pipeline (drop Mastra, plain async, AI SDK v7)

[espetro]

Plan: .agents/plans/2026-07-04-detach-canvas-pipeline.md (WS-2)

Make the AI pipeline Node-API-free so it runs in browser SW.

• Drop Mastra entirely (Node deps block SW). Replace with plain async pipeline.
• Move steps from apps/server/src/workflows/steps/ -> packages/pipeline/src/steps/
• Upgrade AI SDK v6->v7 (npx @ai-sdk/codemod v7). ToolLoopAgent available.
• Reimplement SSE streaming via ReadableStream + data-workflow parts (replace handleWorkflowStream).
• Browser SW runner: ToolLoopAgent per stage, Web Fetch, no fs/child_process.

Effort: L (~4d). Blocks WS-5 (observability instruments this pipeline). Related #8