feat(scribe): merge_snippets — unify found one-off snippets into one canonical

Step 2 of the snippet-merge milestone (#231). The dedup gate only PREVENTS
new near-duplicates; merge is the CURE for the ones already scattered.

- services/snippets.py: merge_snippets(user_id, target_id, source_ids) —
  keep the target's scalar fields (name/when_to_use/signature/language/
  code), union the sources' locations + extra tags onto it (so the survivor
  carries every call site as a location), trash the sources (recoverable),
  re-embed the survivor. Pure merge_snippet_fields() factored out for unit
  testing. Returns (survivor_note, merged_ids).
- mcp/tools/snippets.py: merge_snippets(target_id, source_ids) tool (5th),
  and a create_snippet dedup-path nudge toward merge over a forced copy.
- routes/snippets.py: POST /api/snippets/<id>/merge {source_ids} — share-
  aware (can_write target + every source, rule #78) with a same-owner guard
  (cross-owner merge is out of scope).
- plugin reusing-code skill + MCP _INSTRUCTIONS: point found-duplicates at
  merge as the cure (rule #119 surfaces, not a Scribe rule). plugin.json
  0.1.13 -> 0.1.14 in the same change (the #1040 marketplace-ship lesson).
- Tests: pure merge-helper union/dedup; MCP tool (requires a source,
  survivor+merged_ids, not-found); route handler + 5-tool registration.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Pa2EsuB54BuWQ8GfJq9c7t

src/scribe/mcp/tools/snippets.py

@@ -73,7 +73,11 @@ async def create_snippet(
 
     Returns the created snippet (including a parsed `snippet` field), OR — when a
     near-duplicate snippet already exists and force is false — {"duplicate": true,
-    "existing_id": ..., "message": ...} and nothing is created.
+    "existing_id": ..., "message": ...} and nothing is created. When that happens
+    and it really is the same reusable thing found in another place, prefer
+    merge_snippets(existing_id, [new...]) — or record then merge — to unify them
+    into ONE canonical record (which then carries every call site as a location),
+    rather than forcing a second copy with force=true.
     """
     if not (name or "").strip() or not (code or "").strip():
         raise ValueError("create_snippet requires a non-empty name and code")
@@ -157,6 +161,38 @@ async def update_snippet(
     return data
 
 
+async def merge_snippets(target_id: int, source_ids: list[int]) -> dict:
+    """Unify duplicate/variant snippets INTO one canonical record — the cure for
+    the same reusable thing recorded as several one-offs.
+
+    Keeps the target as the canonical: its name, when-to-use, signature, language
+    and code win. The sources' locations and extra tags are folded in — so the
+    survivor ends up carrying EVERY call site as a location (which is itself the
+    "this is duplicated N times" signal) — and the source records are moved to the
+    trash (recoverable). The survivor is re-embedded so recall stops surfacing the
+    now-merged duplicates.
+
+    Args:
+        target_id: The snippet to keep (the canonical record).
+        source_ids: Snippet ids to fold into the target and retire. Ids that
+            aren't your snippets are skipped; target_id in the list is ignored.
+
+    Returns the merged canonical snippet (with a parsed `snippet` field) plus
+    `merged_ids` — the source ids actually merged and trashed.
+    """
+    uid = current_user_id()
+    ids = [s for s in (source_ids or []) if s != target_id]
+    if not ids:
+        raise ValueError("merge_snippets requires at least one other source_id")
+    result = await snippets_svc.merge_snippets(uid, target_id, ids)
+    if result is None:
+        raise ValueError(f"snippet {target_id} not found")
+    note, merged_ids = result
+    data = snippets_svc.snippet_to_dict(note)
+    data["merged_ids"] = merged_ids
+    return data
+
+
 def register(mcp) -> None:
-    for fn in (list_snippets, create_snippet, get_snippet, update_snippet):
+    for fn in (list_snippets, create_snippet, get_snippet, update_snippet, merge_snippets):
         mcp.tool(name=fn.__name__)(fn)