fix(test): resolve HOME env race condition in parallel tests

- Use get_home_dir() instead of dirs::home_dir() in get_opencode_dir()
  and get_openclaw_dir() to respect CC_SWITCH_TEST_HOME override
- Add CC_SWITCH_TEST_HOME to all TempHome implementations
- Add #[serial] to all with_test_home tests to share serialization
  with other env-mutating tests
- Remove --test-threads=1 workaround from CI

.github/workflows/ci.yml

@@ -96,4 +96,4 @@ jobs:
         run: cargo clippy --manifest-path src-tauri/Cargo.toml -- -D warnings
 
       - name: Run tests
-        run: cargo test --manifest-path src-tauri/Cargo.toml -- --test-threads=1
+        run: cargo test --manifest-path src-tauri/Cargo.toml