CCR event uploads gain hold/drain telemetry

What

• A new uploadHoldMarker setting (gate tengu_ccr_upload_hold_marker, off by default) controls whether the upload-hold telemetry below is included at all.
• When enabled, worker event uploads sent to /worker/events and /worker/internal-events attach an upload_hold object with details such as held, flush_reason, held_count, hold_age_ms, inflight_retry_count, hold_rule, and post_index.
• The upload queue now tracks per-lane counters (posts, rows, max rows per post, failed posts, dropped rows, and more), and fires a new ccr_upload_drain telemetry event with lane, trigger, held events/time, posts, rows, failed posts, dropped rows, and drain duration whenever a batch finishes draining.

Why

This gives better visibility into how CCR (Claude Code Remote, the cloud runner) batches and delays event uploads, which helps diagnose upload backlogs or data loss, without changing behavior while the gate stays off.