- _EXTRACT_SYSTEM now explains 4 transcript entry types and instructs
  LLM to trust tool results over chat, return source/source_context
- _extract_facts builds tool_call_map, appends [Tool call] and
  [Tool result] lines with truncation (500/200 chars)
- Transcript capped at 12k chars (head+tail, drop middle)
- Parse source/source_context from LLM response; map confidence:
  tool_call/auto_discovery=95, user_explicit=90, default=70
- Add TODO comment about deferred semantic deduplication

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>