fix(planning): the plan gate joins step text through embedding_text (#4079)

test_nothing_else_builds_the_embedding_document_itself caught start_planning
building f"{title}\n{body}" inline for the plan gate's candidate text. That is
the embedded-document shape; plan_candidate_text now takes (title, body) pairs
and calls embedding_text, so the candidate moves with the corpus it is ranked
against (#2486).

Also carries the create_task / create_records milestone_id docstring lines from
step 5 (#4080), which share the file.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

src/scribe/services/dedup.py

@@ -767,16 +767,17 @@ async def get_plan_match_threshold(user_id: int) -> float:
 def plan_candidate_text(
     description: str | None = None,
     body: str | None = None,
-    step_texts: list[str] | None = None,
+    steps: list[tuple[str | None, str | None]] | None = None,
 ) -> str:
     """What a plan that doesn't exist yet says about itself, for the gate.
 
     The steps belong in it: a plan passed with steps and no design is still
     recognisable by them, and what its steps say is most of what makes two
-    plans the same plan.
+    plans the same plan. Each step is (title, body), joined by embedding_text
+    like every other record that becomes embedded text (#2486).
     """
     parts = [(description or "").strip(), (body or "").strip()]
-    parts += [t.strip() for t in (step_texts or [])]
+    parts += [embeddings_svc.embedding_text(t, b) for t, b in (steps or [])]
     return "\n\n".join(p for p in parts if p)