Flutter MCP E2E Harness

Golden path for programmatic E2E over the toolkit: packages/harness (flutter_mcp_harness) owns the whole lifecycle — build the app, launch the process, attach to its Dart VM service, and only then drive/assert. It speaks the same ext.mcp.toolkit.* service extensions as the MCP server, as plain Dart. ADR: decisions/0015_flutter_mcp_harness_extraction.mdx.