feat: richer error telemetry — error_message + failed_step fields

Adds error_message (max 200 chars, e.g. "bun test: 3 tests failed")
and failed_step (e.g. "run_tests", "create_pr") to telemetry events.
Schema, ingest function, and local logger all updated. Makes crash
reports actionable instead of just "timeout — 252 occurrences".

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

supabase/migrations/002_community_tier.sql

@@ -1,5 +1,9 @@
 -- gstack community tier schema
--- Adds authenticated backup, benchmarks, and email to the telemetry platform.
+-- Adds authenticated backup, benchmarks, email, and richer error telemetry.
+
+-- Add error context columns to telemetry_events
+ALTER TABLE telemetry_events ADD COLUMN error_message TEXT;
+ALTER TABLE telemetry_events ADD COLUMN failed_step TEXT;
 
 -- Add columns to installations for backup + email + auth identity
 ALTER TABLE installations ADD COLUMN user_id UUID;