feat(shell): flat starship prompt, drop the powerline styling

The powerline look (the `[]` separator glyph + bg-filled segments) didn't
read well. Switch to a flat prompt: bold-accent directory, warn git
branch, subtext git-status/duration, `❯` character — plain colored text,
no background fills, each module carrying its own trailing space so there
are no stray gaps. Also reference cmd_duration in `format` (it was
configured but never shown).

Generated starship.toml verified valid; nix flake check passes. Visual
pass still wants a live terminal.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -364,9 +364,11 @@ close · `SUPER+1..9` workspaces · `Print` region screenshot.
   and yazi fell back to presets. Fixed by adding `group = "git"` to both
   git fetcher entries in `modules/home/yazi.nix` (the git plugin's
   `setup()` only renders the linemode; the fetcher still needs registering).
-- **Starship prompt styling:** drop the powerline look — the current prompt
-  doesn't read well. Rework the `format`/segment styling in
-  `modules/home/shell.nix:59` toward a flat, plain prompt.
+- ✓ **Starship prompt styling:** dropped the powerline look (the `[]`
+  separator glyph + `bg:` fills) for a flat prompt — bold-accent directory,
+  warn git branch, subtext status/duration, `❯` character, all plain
+  colored text (`modules/home/shell.nix`). Also wired in `cmd_duration`,
+  which was configured but never referenced in `format`.
 - ✓ **xfce deprecation warnings:** moved `xfce.exo` → `pkgs.xfce4-exo` and
   the three Thunar plugins to their new top-level names in
   `modules/nixos/file-manager.nix`; the eval warnings are gone.