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>

test/builtin/claude/session_orchestrator_test.dart

@@ -53,6 +53,19 @@ void main() {
     expect(spawnedArgs.single, isNot(contains('--effort')));
   });
 
+  test('the workspace PATH preset reaches the spawned session env (D-106)', () async {
+    final envs = <Map<String, String>?>[];
+    final preset = ClaudeSessionOrchestrator(
+      processFactory: ({required sessionArgs, required cwd, env}) async {
+        envs.add(env);
+        return _FakeProc();
+      },
+      pathPresetFor: (cwd) => cwd == '/repo' ? const ['/opt/go/bin'] : const [],
+    );
+    await preset.spawn(SpawnSpec(id: 'p1', role: 'primary', sessionId: 'p1-uuid', cwd: '/repo'));
+    expect(envs.single?['PATH'], startsWith('/opt/go/bin:'), reason: 'preset dirs lead the delta PATH');
+  });
+
   test('a fresh session gets the skills nudge; resume + fork do not (T-490)', () async {
     String appendPrompt(List<String> args) {
       final i = args.indexOf('--append-system-prompt');