feat(ux): batch UX improvements

- Reverse-video caret in input row (was cyan █ block — collided with
  the cyan ❯ prompt and read as 'unfocused-pane cursor')
- Synchronized-output brackets (DEC mode 2026) around each redraw —
  eliminates tearing on Kitty/WezTerm/foot/recent iTerm; ignored elsewhere
- Two-tone hint row (keys at normal weight, labels dim) gives the eye
  a scan anchor
- Dim header above preview pane (── <change_id> ────) anchors the right
  column to the cursor row's revision
- Light-terminal detection via COLORFGBG: switch cursor-row highlight
  from indexed-237 dark-gray bg to reverse-video on themes where the
  dark bg would be invisible
- Shell wrappers echo the resolved 'jj …' command (dim '$' prefix)
  before running it, so it's visible in scrollback

src/init/nu.nu

@@ -12,5 +12,6 @@ def --env jjf [...args] {
     let cmd = ($result.stdout | str trim)
     if ($cmd | is-empty) { exit 130 }
     [{command: $cmd, start_timestamp: (date now | format date "%+")}] | history import
+    print $"(ansi grey)$(ansi reset) ($cmd)"
     ^sh -c $cmd
 }