Effective Config Validation

When configuration has multiple sources (env vars, CLI flags, socket overrides) and/or platform-specific test compilation, treat only the effective configuration as authoritative.

Apply this standard:

• Precedence first: codify which source wins (e.g., CLI --session > HERDR_SESSION unless HERDR_SOCKET_PATH overrides socket selection).
• Validate conditionally: only validate/parse config values if they will actually be used. Don’t fail early on values that are overridden.
• Lock it with tests: add regression tests for precedence edge cases.
• Platform gating: if a module is already compile-gated for a target, avoid redundant per-test #[cfg]; if needed, gate at the narrowest helper/function scope.

Example (precedence regression shape):

// If no explicit --session is provided and HERDR_SOCKET_PATH is set,
// skip HERDR_SESSION validation because it is not authoritative.
#[test]
fn socket_path_wins_over_invalid_session_env() {
    std::env::set_var("HERDR_SOCKET_PATH", "/tmp/herdr.sock");
    std::env::set_var("HERDR_SESSION", "bad/name");

    // invoke: `herdr workspace list` (or the function that reads config)
    // assert: command proceeds (no validation error), and uses /tmp/herdr.sock
}

This prevents confusing failures from overridden settings and keeps tests compiling cleanly across platforms.