Use `mvdan/sh` as the default command executor shell

Dagu currently launches workflow steps through the platform shell returned by `cmdutil.GetShellCommand("")`. On UNIX this typically resolves to `/bin/sh` (with `-e` appended), while on Windows it falls back to `powershell` or `cmd.exe`. This proposal switches the default executor path to run commands through the Go-native interpreter provided by [`mvdan.cc/sh/v3`](https://github.com/mvdan/sh), providing consistent POSIX shell semantics across platforms while keeping the existing `shell` override for users who need a specific shell binary.

**Requirements**
- Adopt `mvdan/sh` as the default command runner for steps that do not specify `shell`.
- Ensure shell features (pipelines, redirects, here-docs, command substitution, boolean operators, exit status propagation) work consistently on all supported platforms.
- Maintain existing logging, stderr tail capture, environment injection, and working-directory handling in the executor.