feat(shell): prefer Git Bash on Windows

[XingYu-Zhong]

Summary

• prefer Git for Windows Bash for Kun shell commands on Windows
• reuse the same Git Bash discovery for the built-in terminal and Workflow Bash nodes
• preserve workspace cwd, keep safe PowerShell/cmd fallbacks, and document the behavior for v0.2.29

Tests

• npm run typecheck
• npm --prefix kun run typecheck
• npm run lint
• npm run test (4,582 tests)
• npm --prefix kun run test (2,219 tests; 1 platform skip)
• npm run build
• npm run audit:production