wip: wire local leto observability - #21384

Summary

• wire Effect runtime OTLP export to leto via a shared observability layer that is opt-in through OPENCODE_OTLP_BASE_URL
• add tracing and a small amount of logging/annotation around session, snapshot, and formatter workflows to make local latency analysis practical
• disable OTLP during tests by default by requiring explicit URL opt-in, and add follow-up instrumentation for formatter breakdowns

Verification

• bun typecheck
• bun test --timeout 30000 test/effect/run-service.test.ts
• queried local leto APIs to confirm traces/logs, formatter breakdowns, and current hotspots

Follow-up

• validate fresh traces after restart to make sure duplicate span shapes were stale runtime artifacts
• investigate missing-parent trace stitching and eager SessionSummary.summarize work
• consider coalescing Format.file so each touched file formats at most once per step