When a heartbeat run results in HEARTBEAT_OK (or empty/duplicate), the user+assistant
turns are now pruned from the session transcript. This prevents context window
pollution from zero-information exchanges.
Implementation:
- captureTranscriptState(): records transcript file path and size before heartbeat
- pruneHeartbeatTranscript(): truncates file back to pre-heartbeat size
- Called in ok-empty, ok-token, and duplicate cases (same places as restoreHeartbeatUpdatedAt)
This extends the existing pattern where delivery is suppressed and updatedAt is restored
for HEARTBEAT_OK responses - now the transcript is also cleaned up.
Fixes#17804
