feat: flag vendored clones and make the repo filter navigable

Two Repos-tab quality-of-life changes:

- External/vendored clones (repos with zero of your own commits — e.g. an
  upstream cloned for reference and never pushed to) are measured on
  refresh, cached in stats_cache (RepoStatsEntry.authored), shown with a
  dim ⧉ marker prefixed to the name, and hidden by the persistent
  hide_external_clones config toggle (key `x`, the `S` menu, or
  `vulp list --external|--no-external`). A fork you do commit to stays
  yours, so only true reference clones are flagged.

- Inside the `/` filter box, ↑↓ now move through the live-filtered
  results and Enter opens the highlighted repo/project directly, instead
  of the filter only "locking" on Enter with no way to scroll or select.

src/hub/keys.rs

@@ -231,7 +231,26 @@ pub fn tui_phase(state: &mut HubState, existing_term: Option<crate::tui::Tty>) -
                         state.filter_text.clear();
                         state.rebuild_display();
                     }
-                    KeyCode::Enter => { state.filtering = false; }
+                    // Live navigation while the search box is open: arrows move
+                    // the highlight through the filtered results so you can pick
+                    // one without first "locking" the filter (j/k stay literal
+                    // filter input). Enter opens whatever is highlighted.
+                    KeyCode::Down => state.scroll_down(),
+                    KeyCode::Up   => state.scroll_up(),
+                    KeyCode::Enter => match state.tab {
+                        Tab::Repos | Tab::Projects
+                            if state.cur_repo_idx().is_some() || state.cur_proj_idx().is_some() =>
+                        {
+                            // Open the highlighted repo/project directly, keeping
+                            // the filter text applied for when we come back.
+                            state.filtering = false;
+                            state.open_detail = true;
+                            return Ok(PhaseResult::OpenDetail);
+                        }
+                        // Nothing openable under the cursor: just lock the filter
+                        // (exit typing, keep the text) so the full keymap works.
+                        _ => state.filtering = false,
+                    },
                     KeyCode::Backspace => {
                         state.filter_text.pop();
                         state.rebuild_display();