fix(cli): unwrap the walkforward UnknownKnob Debug frame into bare prose

render_bind_error's catch-all Debug-framed UnknownKnob although its
payload is already a fully-prosed message; the variant now renders the
payload alone. The two tests that pinned the frame verbatim repoint to
the prose fragments. Exit 2 unchanged. Completes the #247 defect
family: no Rust identifier reaches a user's stderr on either verb path.

closes #269

crates/aura-cli/src/main.rs

@@ -1920,12 +1920,10 @@ fn override_paths(
 /// (`KindMismatch { .. }` / `MissingKnob("..")`), the two variants the sweep
 /// terminal actually raises past `override_paths`' own pre-flight (which
 /// already rejects an unresolvable axis name as prose before either call
-/// site below ever reaches the terminal). Other variants keep the
-/// established `{:?}` rendering unchanged — in particular `UnknownKnob`
-/// already carries a fully-prosed message string at both call sites here
-/// (wrapped from `override_paths`' own `Result<_, String>`), and pinned
-/// tests (`aura_walkforward_emits_an_unknown_axis_rejection_once` et al.)
-/// depend on that Debug-wrapped shape.
+/// site below ever reaches the terminal). `UnknownKnob` already carries a
+/// fully-prosed message string (wrapped from `override_paths`' own
+/// `Result<_, String>`) — unwrapped here rather than Debug-framed (#269), so
+/// the walkforward path's rejection reaches stderr as bare prose too.
 fn render_bind_error(e: &BindError) -> String {
     match e {
         BindError::MissingKnob(name) => format!(
@@ -1936,6 +1934,7 @@ fn render_bind_error(e: &BindError) -> String {
             "axis {knob}: expected {expected:?}, supplied {got:?} — \
              see `aura sweep <bp> --list-axes`"
         ),
+        BindError::UnknownKnob(msg) => msg.clone(),
         other => format!("{other:?}"),
     }
 }