feat(halachot): Phase 6 — deprecate equivalent_halachot writes post-V41

- halacha_batch_reconcile.py --link now exits with error (V41 freezes
  equivalent_halachot; equivalences live in canonical_id instead)
- link_equivalent_halachot emits DeprecationWarning (callers ≥ Python 3.2)
- SCRIPTS.md already marks --link as deprecated; no further doc change needed

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

mcp-server/src/legal_mcp/services/db.py

@@ -5936,11 +5936,21 @@ def _equiv_order(a: UUID, b: UUID) -> tuple[UUID, UUID]:
 async def link_equivalent_halachot(
     a: UUID, b: UUID, *, cosine: float = 0.0, note: str = "", created_by: str = "",
 ) -> bool:
-    """Record that two halachot (different precedents) state the same principle.
+    """[DEPRECATED since V41] Record a parallel-authority link in equivalent_halachot.
+
+    The canonical_halachot model (V41) supersedes this table — cross-precedent
+    equivalence is now expressed via halachot.canonical_id. This function is kept
+    for historical callers only; no new code should call it. Use
+    ``create_canonical_halacha`` + ``nearest_canonical_halacha`` instead.
 
     Idempotent (symmetric UNIQUE). Returns False and does nothing if a == b or
-    the two belong to the SAME precedent (parallel authority is cross-precedent
-    by definition; within-precedent sameness is the dedup/cluster concern)."""
+    the two belong to the SAME precedent."""
+    import warnings
+    warnings.warn(
+        "link_equivalent_halachot is deprecated since V41 (canonical_halachot). "
+        "Use create_canonical_halacha / nearest_canonical_halacha instead.",
+        DeprecationWarning, stacklevel=2,
+    )
     if a == b:
         return False
     pool = await get_pool()