feat(telemetry): forward cache_write_tokens to conductor

[Wyn2004]

Summary

Forwards cache_write_tokens from the agent's usage counters to Conductor's telemetry hook. usage_to_dict already computed cache_write internally, but the Conductor telemetry hook only ever read cache_read off the usage dict — cache-write activity was silently dropped instead of reaching Conductor's cache-accounting pipeline.

This is the evoflux-side half of a paired change. The Conductor-side half (schema, storage, aggregate rollups, and the UI that displays cache writes and net cache savings) is in evoelsewhere/evo-conductor#6 — merge both together, since Conductor's new column stays 0 for every event until this side ships.

What changed

• app/conductor/constants/telemetry.py: added TelemetryField.CACHE_WRITE_TOKENS, included in TELEMETRY_NUMERIC_TOKEN_FIELDS.
• app/agent/hooks/conductor_telemetry.py: wrap_model_call's success path now passes cache_write_tokens=_counter(usage.get("cache_write")) into _record(...); _record gained the parameter and enqueues it.

Checklist

• Change is scoped to telemetry forwarding only — no behavior change to cost calculation, model calls, or anything user-facing.
• Backward compatible: cache_write_tokens defaults to 0 via _counter(None) when a provider's usage dict has no cache_write key, so nothing breaks for providers that don't report it.
• Existing test suite passes.
• Extended tests/conductor/test_telemetry.py::test_hook_queues_only_safe_model_and_tool_metadata to assert the new field is actually forwarded (cache_write: 15 in the mock usage → TelemetryField.CACHE_WRITE_TOKENS == 15 in the enqueued event), not just that nothing crashed.
• No new dependencies, no config/migration changes on this side.
• Playwright / browser evidence: not applicable — evoflux has no frontend or e2e suite for this path. The visual/behavioral proof that cache-write tokens now flow end-to-end (schema → storage → aggregate → UI) is in the paired Conductor PR's evidence section.

Test evidence

$ uv run pytest tests/conductor/test_telemetry.py -v
...
tests/conductor/test_telemetry.py::test_hook_queues_only_safe_model_and_tool_metadata PASSED
tests/conductor/test_telemetry.py::test_hook_attributes_managed_agent_and_skill_and_closes_request PASSED
tests/conductor/test_telemetry.py::test_hook_attributes_plugin_tools_by_runtime_installation_identity PASSED
tests/conductor/test_telemetry.py::test_hook_closes_failed_request_once_with_resource_attribution PASSED
tests/conductor/test_telemetry.py::test_outbox_preserves_oldest_events_and_reports_queue_health PASSED
tests/conductor/test_telemetry.py::test_client_posts_sanitized_batch PASSED
tests/conductor/test_telemetry.py::test_client_posts_empty_batch_to_refresh_delivery_summary PASSED
tests/conductor/test_telemetry.py::test_managed_skill_usage_is_durable_and_content_free PASSED

8 passed in 7.37s

[khuonghung]

Already Fixed