fix: map error_message and failed_step in telemetry edge function

[sunnnybala]

Summary

• error_message and failed_step fields are captured locally in JSONL by gstack-telemetry-log and the Supabase columns exist (added in 002_tighten_rls.sql), but the edge function's TelemetryEvent interface and rows.push() mapping silently dropped them during remote sync — resulting in perpetually NULL columns.
• Adds both fields to the edge function interface and row mapping with the same pattern as error_class (optional, String() coercion, .slice() truncation).
• Updates the preamble epilogue to pass --error-message and --failed-step flags to gstack-telemetry-log so skills report error context on failures.

Privacy note

Error messages could contain file paths or repo names. The gstack-telemetry-log binary already truncates to 200 chars, and the edge function truncates to 500 chars. Maintainers may want to add redaction in the telemetry-log binary if this is a concern relative to the "no file paths or repo names" telemetry promise.

Test plan

• Verify bun test passes (static validation)
• Verify edge function correctly maps error_message and failed_step to non-NULL values when present in payload
• Verify empty strings map to NULL (JavaScript "" is falsy, shell [ -n "" ] is false)
• Verify existing fields are unaffected

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