Append a stable transcript-file hint (path + line-count snapshot) to
every conversation summary, so after compaction the model can read
the uncompacted transcript on disk.
The hint is appended exactly once at summary-creation time and stored
on round.summary / turn metadata. Subsequent renders replay that
string byte-identically, preserving Anthropic prompt cache hits even
as the transcript keeps growing.
Covers all three summarization paths:
- Full / Simple via ConversationHistorySummarizer.summarizeHistory()
- Inline background via agentIntent.ts _startBackgroundSummarization
(flushes the transcript before snapshotting the line count so the
baked count matches the on-disk file)
Shared via new exported helper appendTranscriptHintToSummary.
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