Summary

Adds a dated observability roadmap spec for Conner to review.

The doc captures:

• the restarted backend/session tracing split from the broad origin/carey/tkai-2-session-tracing branch
• Grafana Faro as a separate frontend RUM/browser-error telemetry PR
• privacy requirements, verification expectations, and the open correlation question

Test plan

• CI green
• Inspected final docs-only diff
• Ran git status --short --branch
• Checked repo scripts; no markdown lint script was present

Checklist

• If adding/changing D1 schema: migration file added in packages/worker/migrations/ (not applicable; docs-only)
• If changing packages/runner/ or docker/: IMAGE_BUILD_VERSION bumped in backend/images/base.py (not applicable; docs-only)
• If adding a new plugin with actions/channels: make generate-registries run and output committed (not applicable; docs-only)