fix(snippets): close the recall-surface gaps found reviewing the Drafter

Four defects from the 2026-07-25 review of the recall (#227) and merge (#231)
milestones. The theme: a snippet could be recorded but not fully corrected, and
the agent and web surfaces had drifted apart.

- #2076 language was mis-derived from the first caller tag, so a snippet created
  with tags and no language read that tag back as its language — corrupting the
  tag set and the code fence on the next update. Only the FIRST tag can carry
  the language, since compose_tags emits [language, "snippet", *caller].
- #2077 MCP update_snippet mapped "" to "unchanged", so no field could ever be
  cleared and no snippet detached from its project. Now an omitted field is left
  alone, an empty string clears, and project_id follows the -1 = detach
  convention. A service-level UNSET sentinel keeps None available as the clear.
- #2078 surface parity: adds delete_snippet (MCP had none, so a wrong snippet
  could not be retired by the agent that recorded it), locations on MCP create
  and update, system_ids through the REST routes and the editor, and the
  near-duplicate gate on REST create with a "record it anyway" escape.
- #2079 project scoping: list_snippets takes project_id through the service, the
  MCP tool and the REST route, defaulting to every project — reaching across
  projects is the point when the helper you need was written elsewhere.

Sharing the list across owners is deliberately NOT in here: query_knowledge is
shared with the Knowledge browse surface, so widening it changes behaviour well
beyond snippets. Left open on #2079 for a scope decision.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RLwAaV4DQEmVyn496HnEvt

src/scribe/mcp/server.py

@@ -223,7 +223,10 @@ project_id, system_ids) so a later session is offered it. Make when_to_use sharp
 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).
+duplicates). Keep the record honest: a snippet whose details have gone stale can
+be corrected with update_snippet (an empty string clears a field), and one that
+is wrong or obsolete should be retired with delete_snippet — a bad snippet keeps
+being offered as prior art, which costs more than none at all.
 
 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