feat(quick-switcher): derive workspace dir from active tab

The explorer panel's explorerCurrentDir is hardcoded to null, so
the Quick Switcher's workspace toggle was a no-op. Rather than fix
the broken explorer flow, derive the workspace from the active
tab's filePath's parent directory — the most reliable source we
have without restructuring that panel.

deriveWorkspaceDir() handles POSIX and Windows paths (including
mixed separators and trailing separators), returns null for
untitled / empty inputs so the workspace toggle silently stays
off in those cases.

10 new tests cover POSIX, Windows, mixed-separator, trailing
separator, UNC, and invalid input paths.

Amit Haridas

tests/workspace-dir-resolver.test.js

@@ -0,0 +1,54 @@
+/**
+ * @jest-environment node
+ *
+ * Workspace dir resolver — derive parent dir from a file path.
+ */
+
+const { deriveWorkspaceDir } = require('../src/quick-switcher/workspace-dir-resolver');
+
+describe('deriveWorkspaceDir', () => {
+  test('returns parent dir for a POSIX path', () => {
+    expect(deriveWorkspaceDir('/home/user/notes/readme.md')).toBe('/home/user/notes');
+  });
+
+  test('returns parent dir for a Windows path', () => {
+    expect(deriveWorkspaceDir('C:\\Users\\me\\notes\\readme.md')).toBe('C:\\Users\\me\\notes');
+  });
+
+  test('returns parent dir for a nested POSIX path', () => {
+    expect(deriveWorkspaceDir('/a/b/c/d/e.md')).toBe('/a/b/c/d');
+  });
+
+  test('auto-detects separator when path contains both', () => {
+    // A path with both — backslash takes precedence (Windows convention)
+    expect(deriveWorkspaceDir('C:\\foo/bar.txt')).toBe('C:\\foo');
+  });
+
+  test('returns null for a root-level POSIX file', () => {
+    expect(deriveWorkspaceDir('readme.md')).toBeNull();
+  });
+
+  test('returns null for an empty string', () => {
+    expect(deriveWorkspaceDir('')).toBeNull();
+  });
+
+  test('returns null for null / undefined / non-string', () => {
+    expect(deriveWorkspaceDir(null)).toBeNull();
+    expect(deriveWorkspaceDir(undefined)).toBeNull();
+    expect(deriveWorkspaceDir(42)).toBeNull();
+    expect(deriveWorkspaceDir({})).toBeNull();
+  });
+
+  test('returns null for a single-character filename', () => {
+    expect(deriveWorkspaceDir('/x')).toBeNull();
+    expect(deriveWorkspaceDir('\\x')).toBeNull();
+  });
+
+  test('preserves trailing separators', () => {
+    expect(deriveWorkspaceDir('/home/user/notes/')).toBe('/home/user');
+  });
+
+  test('handles UNC-style Windows path', () => {
+    expect(deriveWorkspaceDir('\\\\server\\share\\file.md')).toBe('\\\\server\\share');
+  });
+});