feat(scribe): add snippet recall — note_type='snippet' service + MCP tools

Record reusable functions/components once so they surface via the existing
semantic search + title-first auto-inject, instead of re-solving as one-offs.

A snippet is a Note with note_type='snippet' (no schema change): note_type is
free-text, and semantic_search_notes never filters by type, so snippets join
the recall/auto-inject pool the moment they're embedded. Structured fields
(name/language/signature/location/when_to_use/code) are stored via a body
convention — title = "name — when to use" (what auto-inject surfaces), language
+ "snippet" as tags, templated markdown body — keeping storage swappable later
without changing the tool/UI contract.

- services/snippets.py: compose/parse helpers + create/get/list/update wrappers
  over notes_svc (dedup + System association reused).
- mcp/tools/snippets.py: list_snippets / create_snippet / get_snippet /
  update_snippet, registered in tools/__init__.py.
- unit tests for the serialize/parse round-trip and the MCP tool surface.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Pa2EsuB54BuWQ8GfJq9c7t

src/scribe/mcp/tools/__init__.py

@@ -5,7 +5,7 @@ to a FastMCP instance. `register_all(mcp)` is the single entry point called
 from `mcp.server.build_mcp_server`.
 """
 from scribe.mcp.tools import (
-    milestones, notes, processes, projects, recent, repos, rulebooks, search, systems, tags, tasks, trash,
+    milestones, notes, processes, projects, recent, repos, rulebooks, search, snippets, systems, tags, tasks, trash,
 )
 
 
@@ -21,5 +21,6 @@ def register_all(mcp) -> None:
     recent.register(mcp)
     repos.register(mcp)
     processes.register(mcp)
+    snippets.register(mcp)
     rulebooks.register(mcp)
     trash.register(mcp)