Add WorkdirRoot configuration for custom work directories (#27)

- [x] Add `workdir_root` field to `RunParams` struct
- [x] Add `workdir_root` method to `Builder` API  
- [x] Modify `TestEnvironment::new()` to accept optional root directory
- [x] Update `TestEnvironment::new()` to use
`tempfile::TempDir::new_in()` when root is specified
- [x] Add validation for root directory existence and writability
- [x] Add tests for the new workdir_root functionality
- [x] Update documentation and examples
- [x] Address PR feedback: remove unnecessary files and improve tests
- [x] Fix code formatting with cargo fmt
- [x] Add rustfmt.toml configuration and update CLAUDE.md
- [x] Improve basic test to verify actual workdir usage

## Recent Changes

**Enhanced basic functionality test:**
- Modified `test_workdir_root_basic_functionality` to induce a failure
and use `preserve_work_on_failure(true)`
- Test now verifies that the workdir was actually created in the custom
root directory
- Validates that test files exist in the preserved workdir with expected
content
- **Removed** redundant `test_workdir_root_with_preserve_work` test as
the functionality is now covered by the improved basic test

The test suite is now more meaningful and comprehensive, with 4 focused
tests that provide real validation of the workdir_root functionality:
- Basic functionality with actual workdir verification
- Error handling for nonexistent directories  
- Error handling for non-directory paths
- API chaining validation

All tests pass and the implementation properly validates that custom
workdir roots are used as expected.

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src/lib.rs

@@ -120,6 +120,18 @@ fn run(params: &mut RunParams, test_data_glob: &str) -> Result<()> {
 ///     true
 /// }
 /// ```
+///
+/// ### With Custom Work Directory Root
+/// ```no_run
+/// use testscript_rs::testscript;
+///
+/// // Use a custom location for test working directories
+/// testscript::run("testdata")
+///     .workdir_root("/tmp/my-app-tests")
+///     .preserve_work_on_failure(true)  // Combine features
+///     .execute()
+///     .unwrap();
+/// ```
 pub struct Builder {
     dir: String,
     params: RunParams,
@@ -229,6 +241,41 @@ impl Builder {
         self
     }
 
+    /// Set the root directory for test working directories
+    ///
+    /// When specified, test directories will be created inside this root directory
+    /// instead of the system default temporary directory. This is useful for:
+    /// - **Debugging**: Use a known location for test directories
+    /// - **Performance**: Use faster storage (e.g., tmpfs on Linux)
+    /// - **CI environments**: Use specific temp locations
+    /// - **Security**: Isolate test directories to specific paths
+    ///
+    /// # Arguments
+    /// * `root` - The directory path where test working directories should be created
+    ///
+    /// # Examples
+    /// ```no_run
+    /// use testscript_rs::testscript;
+    ///
+    /// // Use custom location for test directories
+    /// testscript::run("testdata")
+    ///     .workdir_root("/tmp/my-app-tests")
+    ///     .preserve_work_on_failure(true)  // Combine with other features
+    ///     .execute()
+    ///     .unwrap();
+    /// ```
+    ///
+    /// This creates test directories like `/tmp/my-app-tests/testscript-abc123/`.
+    ///
+    /// # Notes
+    /// - The root directory must exist and be writable
+    /// - If not specified, uses the system default temporary directory
+    /// - Each test still gets its own isolated subdirectory
+    pub fn workdir_root<P: Into<std::path::PathBuf>>(mut self, root: P) -> Self {
+        self.params = self.params.workdir_root(root);
+        self
+    }
+
     /// Execute all test scripts in the configured directory
     ///
     /// This will discover all `.txt` files in the directory and run them as test scripts.