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_mcp_tool_snippets.py

@@ -82,7 +82,38 @@ async def test_update_snippet_missing_raises():
             await update_snippet(123, name="x")
 
 
-def test_register_attaches_four_tools():
+@pytest.mark.asyncio
+async def test_merge_snippets_requires_a_source():
+    from scribe.mcp.tools.snippets import merge_snippets
+    with pytest.raises(ValueError):
+        await merge_snippets(target_id=1, source_ids=[])
+    with pytest.raises(ValueError):
+        await merge_snippets(target_id=1, source_ids=[1])  # only self → nothing to merge
+
+
+@pytest.mark.asyncio
+async def test_merge_snippets_returns_survivor_and_merged_ids():
+    survivor = _fake_snippet()
+    with patch("scribe.services.snippets.merge_snippets",
+               AsyncMock(return_value=(survivor, [2, 3]))) as mock_merge:
+        from scribe.mcp.tools.snippets import merge_snippets
+        out = await merge_snippets(target_id=1, source_ids=[2, 3, 1])
+    assert out["merged_ids"] == [2, 3]
+    assert out["snippet"]["name"] == "debounce"
+    # target_id passed through; self-id filtered out of the source list.
+    assert mock_merge.await_args.args[1] == 1
+    assert mock_merge.await_args.args[2] == [2, 3]
+
+
+@pytest.mark.asyncio
+async def test_merge_snippets_not_found_raises():
+    with patch("scribe.services.snippets.merge_snippets", AsyncMock(return_value=None)):
+        from scribe.mcp.tools.snippets import merge_snippets
+        with pytest.raises(ValueError):
+            await merge_snippets(target_id=1, source_ids=[2])
+
+
+def test_register_attaches_all_tools():
     from scribe.mcp.tools import snippets
     names: list[str] = []
 
@@ -97,4 +128,5 @@ def test_register_attaches_four_tools():
     snippets.register(FakeMcp())
     assert set(names) == {
         "list_snippets", "create_snippet", "get_snippet", "update_snippet",
+        "merge_snippets",
     }