ci: refactor docs-validation workflow to use environment variables

- Add env section with WORKSPACE_BASE, REPO_URL, STANDARD_DIRS
- Replace hardcoded paths with environment variable references
- Extract standard directory creation to dedicated step
- Remove duplicate directory creation from lint steps
- Remove duplicate error message in documentation structure check
- Update all workspace paths to use WORKSPACE_BASE variable
- Update repository URL to use REPO_URL variable

test: fix import path in test_import_surface.py after utils reorgan

tests/verification/test_import_surface.py

@@ -74,7 +74,7 @@ def test_agent_coordinator_wrapper_bootstrap(monkeypatch) -> None:
 
     with monkeypatch.context() as m:
         m.setattr(os, "execvp", fake_execvp)
-        m.setattr(aitbc_paths, "ensure_dir", lambda path: path)
+        m.setattr(aitbc.utils.paths, "ensure_dir", lambda path: path)
         m.setenv("AITBC_ENV_FILE", "placeholder")
         m.setenv("AITBC_NODE_ENV_FILE", "placeholder")
         m.setenv("PYTHONPATH", "placeholder")