WS-5: Observability (OTel spans, OTLP sink, token aggregation)

[espetro]

Plan: .agents/plans/2026-07-04-detach-canvas-pipeline.md (WS-5)

Replace Mastra's workflow observability (lost when Mastra dropped in WS-2) with standard OTel.

• @opentelemetry/api span creation in plain-async pipeline steps (packages/pipeline)
• @opentelemetry/sdk-node for desktop runtime; fetch-based OTLP exporter for browser SW
• otelcol-contrib for local dev -> Tempo/console
• Token usage aggregation per step (extend telemetry.ts in packages/core)
• /api/metrics endpoint (Prometheus-compatible)

Stack: standard OTel + OTLP. NOTE: WS-5 depends on WS-2 (instruments the plain-async pipeline, not Mastra steps).

Effort: M (~2d). Depends on WS-2.