fix(env): harden the PATH preset per review (T-511)

Three holes from the T-511 adversarial review pass:

- An entry containing the PATH separator smuggled extra tokens into
  the joined PATH — a stray trailing ':' yields an EMPTY token, which
  POSIX shells resolve as CWD (the dot-in-PATH hazard). The CLI verb
  and the settings control now reject such entries, and applyPathPreset
  skips malformed stored values that predate the check.
- The gitdir pointer a worktree resolution follows is repo-controlled
  text; the resolved main root is now validated (must hold a real
  .git directory) before its preset key is trusted, so a crafted
  pointer can't alias an arbitrary path's preset.
- A pane spawned with a cwd below the workspace root hashed the
  subdirectory and silently missed the workspace preset; the lookup
  now keys any in-workspace cwd to the workspace root
  (presetLookupRoot).

Also: the Add button pairs buttonBackground with its own
buttonHoverBackground token instead of borrowing the list-item hover
token, and the hosted-Claude leg gains an end-to-end orchestrator test
(preset lookup → spawn env).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/main.dart

@@ -63,7 +63,7 @@ import 'package:clide/src/daemon/search_commands.dart';
 import 'package:clide/src/editor/registry.dart' show EditorRegistry;
 import 'package:clide/src/git/client.dart';
 import 'package:clide/src/cli/argv_dispatch.dart';
-import 'package:clide/src/env/path_preset.dart' show applyPathPreset, pathPresetKey, presetDirsFrom;
+import 'package:clide/src/env/path_preset.dart' show applyPathPreset, pathPresetKey, presetDirsFrom, presetLookupRoot;
 import 'package:clide/src/env/shell_env.dart' show primeLoginShellPath, resolvedToolPath;
 import 'package:clide/src/env/supporter_binaries.dart';
 import 'package:clide/widgets/src/icons/phosphor_glyphs.g.dart' show kPhosphorGlyphs;
@@ -322,7 +322,7 @@ Future<void> main() async {
         final settings = kernelSettings;
         final base = resolvedToolPath();
         if (settings == null) return base;
-        return applyPathPreset(base, presetDirsFrom((k) => settings.get<Object>(k), cwd ?? workRoot.path));
+        return applyPathPreset(base, presetDirsFrom((k) => settings.get<Object>(k), presetLookupRoot(cwd, workRoot.path)));
       },
     );
     // D-6 parity (T-219, D-83): make the tabs the user sees in the GUI