Finds the outermost project root by walking up from file_path.
For monorepos with nested .git dirs (e.g. mono/backend/.git + mono/frontend/.git),
returns the outermost ancestor containing .git, a workspace marker, or a known
monorepo config file — so the whole monorepo is treated as one project.
Returns the project root for file_path, falling back to cwd if none found.
Checks LEAN_CTX_PROJECT_ROOT env var and config.toml project_root first.
Logs a warning when the fallback is a broad directory (home, root).