feat(mcp): tools/ package + fable_search

Establishes the tool pattern: each tool module exposes register(mcp),
register_all() aggregates them, build_mcp_server() calls register_all.

fable_search mirrors the existing fable-mcp contract (q/content_type/limit
in; {results, total} out) but calls services.embeddings.semantic_search_notes
directly instead of going over HTTP. User comes from mcp.current_user_id().

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/fabledassistant/mcp/tools/__init__.py

@@ -0,0 +1,12 @@
+"""MCP tool implementations.
+
+Each tool module exposes a `register(mcp)` function that attaches its tools
+to a FastMCP instance. `register_all(mcp)` is the single entry point called
+from `mcp.server.build_mcp_server`.
+"""
+from fabledassistant.mcp.tools import search
+
+
+def register_all(mcp) -> None:
+    """Register every tool module's tools on the given FastMCP instance."""
+    search.register(mcp)