feat(mcp): scaffold in-app FastMCP package

Empty FastMCP instance with the post-pivot instructions block. Tools
get registered in phases 2 and 3; ASGI mounting + bearer-auth comes
in task 1.4.

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

src/fabledassistant/mcp/__init__.py

@@ -0,0 +1,8 @@
+"""In-app MCP server. Exposes Fabled Scribe data via the MCP streamable-HTTP transport.
+
+Auth uses the existing `api_keys` table: a Bearer token in the Authorization
+header resolves to a user, and every tool call acts on that user's data only.
+"""
+from fabledassistant.mcp.server import build_mcp_server, mount_mcp
+
+__all__ = ["build_mcp_server", "mount_mcp"]