docs: rewrite telemetry guides to be terse and accurate

- docs/TELEMETRY.md: 717 → 123 lines. Drop duplicate privacy sections,
  long examples, architecture deep-dive. Fix the inaccurate "disabled by
  default" claim. Cross-reference IDBSQLClient.ts JSDoc for defaults so
  the doc can't drift.
- spec/telemetry-design.md: 2538 → 125 lines. Drop the implementation
  checklist, exhaustive proto field table, and inline class bodies that
  mirrored lib/telemetry/*.ts. Keep architecture diagram, per-component
  responsibilities, export lifecycle, privacy/error/shutdown invariants.
- spec/telemetry-test-completion-summary.md: 648 → 47 lines. Collapse
  per-component prose into a single table, drop verbose
  exit-criteria-verification and quality-metrics sections.

Co-authored-by: Isaac

Author: samikshya-db