graphiti_service: expose custom_extraction_instructions on /episodes/bulk; add saga on /episodes

- BulkEpisodeRequest: new optional custom_extraction_instructions field
  with comment noting graphiti-core inserts it into extract_nodes/extract_edges
  prompts only, NOT dedupe prompts (verified by reading prompts directory)
- EpisodeRequest: new optional saga field, plumbed through to add_episode
  for upcoming Stage 3 single-episode pathway
- Both handlers use conditional kwargs construction so existing callers
  see no behavioral change

Phase A item 1 of three. Items 2 (stage2_worker) and 3 (stage3_worker) follow.

scripts/graphiti_service.py

@@ -92,6 +92,12 @@ class BulkEpisodeRequest(BaseModel):
     episodes: list[BulkEpisodeItem]
     group_id: str | None = None
     saga: str | None = None
+    # Batch-level extraction guidance. graphiti-core inserts this into the
+    # entity-extraction and edge-extraction prompts only — NOT into dedup
+    # prompts. Use to bias *what* gets extracted, not *how* dedup runs.
+    # Verified 2026-05-01 by reading extract_nodes.py, extract_edges.py,
+    # dedupe_nodes.py, dedupe_edges.py in graphiti-core.
+    custom_extraction_instructions: str | None = None
 
 
 class EpisodeRequest(BaseModel):
@@ -101,6 +107,7 @@ class EpisodeRequest(BaseModel):
     timestamp: str | None = None
     group_id: str | None = None
     custom_extraction_instructions: str | None = None
+    saga: str | None = None
 
 @app.get("/health")
 async def health():
@@ -112,7 +119,7 @@ async def add_episode(req: EpisodeRequest):
     from graphiti_core.nodes import EpisodeType
     try:
         ref_time = datetime.fromisoformat(req.timestamp) if req.timestamp else datetime.now()
-        await g.add_episode(
+        kwargs = dict(
             name=req.name,
             episode_body=req.content,
             source=EpisodeType.text,
@@ -121,6 +128,11 @@ async def add_episode(req: EpisodeRequest):
             group_id=req.group_id or GROUP_ID,
             custom_extraction_instructions=req.custom_extraction_instructions,
         )
+        # Saga is supported on graphiti-core add_episode but kept optional
+        # so older callers don't need to know about it.
+        if req.saga is not None:
+            kwargs["saga"] = req.saga
+        await g.add_episode(**kwargs)
         return {"ok": True}
     except Exception as e:
         log.error(f"Episode ingestion failed: {e}\n{traceback.format_exc()}")
@@ -142,11 +154,16 @@ async def add_episodes_bulk(req: BulkEpisodeRequest):
             reference_time=ref_time,
         ))
     try:
-        result = await g.add_episode_bulk(
+        kwargs = dict(
             bulk_episodes=raw_episodes,
             group_id=req.group_id or GROUP_ID,
             saga=req.saga or None,
         )
+        # Pass-through only when set, so callers that don't supply
+        # instructions get graphiti-core's default behavior unchanged.
+        if req.custom_extraction_instructions is not None:
+            kwargs["custom_extraction_instructions"] = req.custom_extraction_instructions
+        result = await g.add_episode_bulk(**kwargs)
         return {"ok": True, "count": len(raw_episodes)}
     except Exception as e:
         log.error(f"Bulk ingestion failed: {e}\n{traceback.format_exc()}")