Moved the BM25 discovery index out of mcp/discoverable-tool-metadata.ts
into tool-discovery/tool-index.ts and generalized it to work over any
AgentTool source (built-in, MCP, extension, custom) instead of only MCP.
Added DiscoverableTool with explicit source and summary fields. Kept
the MCP-typed helpers (collectDiscoverableMCPTools, searchDiscoverable
MCPTools, etc.) as thin re-exports from the original path so existing
callers continue to work unchanged.
Tests cover the generic index, source filtering, summary fallback, and
back-compat with the legacy MCP-only API.