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

tests/test_services_snippets.py

@@ -122,6 +122,22 @@ def test_parse_legacy_single_location_line_still_works():
     assert (got["repo"], got["path"], got["symbol"]) == ("scribe", "a.py", "f")
 
 
+def test_merge_snippet_fields_unions_locations_and_tags():
+    target_fields = {"language": "py", "locations": [{"repo": "a", "path": "a.py", "symbol": "f"}]}
+    src1 = ({"language": "py", "locations": [{"repo": "b", "path": "b.py", "symbol": "g"}]},
+            ["py", "snippet", "helper"])
+    src2 = ({"language": "py", "locations": [{"repo": "a", "path": "a.py", "symbol": "f"}]},  # dup loc
+            ["snippet"])
+    locs, extra = s.merge_snippet_fields(target_fields, ["py", "snippet", "core"], [src1, src2])
+    # target location first, then unique source locations; dup dropped.
+    assert locs == [
+        {"repo": "a", "path": "a.py", "symbol": "f"},
+        {"repo": "b", "path": "b.py", "symbol": "g"},
+    ]
+    # extra tags unioned, language + "snippet" markers excluded.
+    assert extra == ["core", "helper"]
+
+
 def test_snippet_to_dict_includes_parsed_fields():
     class FakeNote:
         title = "debounce — rate-limit"