Add a `platform` variable to the MiniJinja context when rendering task
commands. This variable contains the best platform for the environment
in which the task will run (e.g., linux-64, osx-arm64, win-64).

This enables users to create platform-specific tasks without hardcoding
platform names:

```toml
[tasks]
build = { cmd = "cargo build --target {{ platform }}", args = [] }
download-binary = {
    cmd = "curl -O https://example.com/binary-{{ platform }}.tar.gz",
    args = []
}
```

The platform is automatically determined from the environment's
best_platform() method, which respects the environment configuration
and handles platform fallback (e.g., osx-64 on Apple Silicon when
osx-arm64 is not available).

Changes:
- Add platform parameter to TemplateString::render() and related methods
- Pass environment's best_platform() from ExecutableTask to render calls
- Add tests for platform variable rendering
- Update documentation with platform variable usage examples

Update test code in pixi_core and pixi crates to use the new
TaskRenderContext API instead of passing None directly.

Add new system variables available in task templates under the pixi namespace:
- pixi.environment - The name of the current environment
- pixi.manifest-path - Absolute path to the manifest file
- pixi.version - The version of pixi
- pixi.is_win, pixi.is_unix, pixi.is_linux, pixi.is_osx - Platform detection booleans

Note: The platform detection booleans are accessible as pixi.is_win, etc.
rather than pixi.platform.is_win to avoid minijinja type complexity.

Updated TaskRenderContext to optionally include environment name and manifest path.

Update the documentation to include all available pixi template variables
in a clear markdown table format. Added examples showing how to use:
- Platform detection booleans for conditional logic
- Environment-aware tasks
- Manifest path access with bracket notation

Change pixi.manifest-path to pixi.manifest_path to avoid issues with
dashes being interpreted as minus signs in MiniJinja. This is consistent
with the variable naming convention and avoids the need for bracket notation.

- Changed manifest_path from Option<&Path> to Option<PathBuf> to avoid lifetime issues
- Added render_context() method to ExecutableTask for consistent context construction
- Removed new() and with_args() methods, using struct initialization syntax instead
- Ensures environment name and manifest path are properly passed in all contexts
- All tests passing