- Trim metrics from 18 to 6, removing counters that duplicated info
available from existing metrics or provided little operational signal.
- Inline single-use constants.
- Re-add hidden PubsubFlushInterval and PubsubQueueSize config knobs
to ChatConfig for tuning without code changes.
- Remove dead ErrBatchingPubsubQueueFull export (never returned).
- Remove unreachable nil branch in batchFlushStage.
- Propagate resetErr through flushBatch even when delegate replay
succeeds, so drain reports the broken sender state.
- Annotate sender field with goroutine-safety invariant.
- Regenerate metrics docs.
Ethan Dickson
de1a317890