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/server.py

@@ -220,7 +220,10 @@ one-off; (2) the moment you build or notice something reusable, record it with
 create_snippet(name, code, when_to_use, language, signature, repo, path, symbol,
 project_id, system_ids) so a later session is offered it. Make when_to_use sharp
 — it becomes the title, which is what a recall menu shows. Edit an existing one
-with update_snippet rather than recording a second copy.
+with update_snippet rather than recording a second copy; when the same reusable
+thing already exists as several one-offs, unify them into one canonical record
+with merge_snippets (it folds every call site in as a location and trashes the
+duplicates).
 
 When developing Scribe itself, honor its multi-user sharing ACL: scope every
 read and mutation of user data by owner + shares — never assume a single