Add INIT_CWD environment variable support

[Copilot]

Implements the INIT_CWD environment variable following npm's specification, which holds the full path where vite-plus was invoked.

Background

As documented in the npm CLI docs:

Scripts are run from the root of the package folder, regardless of what the current working directory is when npm is invoked. If you want your script to use different behavior based on what subdirectory you're in, you can use the INIT_CWD environment variable, which holds the full path you were in when you ran npm run.

This enables scripts to determine the original invocation context and adapt their behavior accordingly.

Implementation

The implementation follows existing patterns in the codebase:

• Static Variable: Uses LazyLock<Mutex<Option<AbsolutePathBuf>>> consistent with CURRENT_EXECUTION_ID
• Automatic Injection: Added to DEFAULT_PASSTHROUGH_ENVS for automatic availability in all tasks
• Thread-Safe: Proper synchronization for concurrent task execution
• Zero Configuration: Works automatically without any setup required

Usage Example

# From workspace root
$ cd /path/to/workspace && vite run build
# Inside task: INIT_CWD=/path/to/workspace

# From subdirectory
$ cd /path/to/workspace/packages/app && vite run build  
# Inside task: INIT_CWD=/path/to/workspace/packages/app

Tasks can now use $INIT_CWD to access the original invocation directory, enabling context-aware script behavior.

Testing

• Added comprehensive unit test test_init_cwd_environment_variable
• Verified integration with real workspace scenarios
• All existing tests pass, ensuring no regressions

Fixes #74.

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