Function codineer_runtime_dir 

pub fn codineer_runtime_dir(cwd: &Path) -> PathBuf

Returns the .codineer/ directory to use for runtime artifacts (sessions, agents, todos, sandbox dirs, etc.).

Uses the nearest initialized project’s .codineer/ (found by walking up the ancestor chain). Falls back to ~/.codineer/ when no project is initialized; cwd/.codineer/ as last resort.