Related Issue

No linked issue. This is internal telemetry hygiene; the motivation is in the next section.

Problem

Telemetry property keys had drifted into an inconsistent mix of camelCase and
snake_case across events, and a few events carried redundant or sparse fields:

• compaction_finished / compaction_failed and micro_compaction_finished
  used camelCase (tokensBefore, compactedCount, retryCount, …) while the
  rest of the telemetry surface (subagent_name, run_in_background,
  duration_ms, …) is snake_case.
• update_prompted emitted both current/latest and
  current_version/target_version for the same values.
• Client attribution on session_started was a sparse key set (empty fields
  omitted), so the two producers (daemon core-impl and single-process
  kimi-harness) could emit differently-shaped events.

Telemetry is untyped (TelemetryProperties = Record<string, …>), so none of
this is caught by the compiler; the contract lives in the downstream consumers.
Keeping the key schema consistent and stable is what makes those consumers
tractable.

What changed

• Renamed the remaining camelCase telemetry keys to snake_case
  (tokens_before, tokens_after, compacted_count, retry_count,
  thinking_level, error_type, input_tokens / output_tokens, and the
  micro-compaction config / effect keys).
• Emit a fixed client-attribution key set (client_id / client_name /
  client_version / ui_mode, null when the client did not provide a field)
  from both session_started producers so they share one schema.
• Removed the duplicate current / latest keys on update_prompted and the
  redundant ui_mode on server_started (the server's boot UI mode is now
  covered by the client-attribution ui_mode).
• Additive: login now carries method: 'oauth' (matching the existing
  method: 'api_key' path), and question_answered carries an answered
  count.

A few fields changed shape, not just name, worth flagging explicitly:

• compaction_finished no longer emits the custom instruction, and the
  per-component token breakdown (inputCacheRead / inputCacheCreation) was
  folded into a single input_tokens (= inputTotal).
• The tool / video_upload error event moved from error (full message) to
  error_type (error class name), matching the convention already used by
  crash, api_error, and compaction_failed (low-cardinality type on
  telemetry; the message stays on hooks / logs).

The persisted apply_compaction record fields (CompactionResult.tokensBefore
etc.) are intentionally left camelCase — they are consumed by external
projectors and are a separate channel from telemetry.

Compatibility

This renames and remaps telemetry property keys that downstream dashboards and
alerts may reference. Producers and consumers should be coordinated before this
lands.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works. (Existing telemetry tests were updated to assert the new schema.)
• Ran gen-changesets skill, or this PR needs no changeset. (Telemetry-only change — no changeset.)
• Ran gen-docs skill, or this PR needs no doc update. (Internal telemetry schema — no user-facing docs to update.)