transcript progress: capnp append-only log replaces stub nodes

Add TranscriptSegment capnp schema and append-only log for tracking
which transcript segments have been mined by which agents. Replaces
the old approach of creating stub nodes (_observed-transcripts,
_mined-transcripts, _facts-) in the main graph store.

- New schema: TranscriptSegment and TranscriptProgressLog
- Store methods: append_transcript_progress, replay, is_segment_mined,
  mark_segment_mined
- Migration command: admin migrate-transcript-progress (migrated 1771
  markers, soft-deleted old stub nodes)
- Progress log replayed on all Store::load paths

Also: revert extractor.agent to graph-only (no CONVERSATIONS),
update memory-instructions-core with refine-over-create principle.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

poc-memory/schema/memory.capnp

@@ -122,3 +122,18 @@ struct AgentVisit {
 struct AgentVisitLog {
   visits @0 :List(AgentVisit);
 }
+
+# Transcript mining progress — separate append-only log.
+# Tracks which segments of which transcripts have been processed,
+# by which agent, so we never re-mine the same content.
+
+struct TranscriptSegment {
+  transcriptId  @0 :Text;     # session UUID (filename stem)
+  segmentIndex  @1 :UInt32;   # compaction segment index within transcript
+  agent         @2 :Text;     # "observation", "experience", "fact"
+  timestamp     @3 :Int64;    # unix epoch seconds when mining completed
+}
+
+struct TranscriptProgressLog {
+  segments @0 :List(TranscriptSegment);
+}