feat(template): add dry-run install previews

[phucnguyen1707]

Summary

Supersedes #209 after #205 changed the template safety code. This rebased version preserves #205's symlink and special-file protections.

• add moshcode template install <source> --dry-run to preview every create and overwrite without changing the target
• support --force --dry-run for a complete overwrite preview while preserving the existing conflict behavior
• reject unknown options and missing --into values before fetching or writing anything
• remove the complete temporary workspace after remote tarball installs and previews
• document the option and add Bash, Zsh, Fish, and PowerShell completion

Tests

• node --test --test-concurrency=1 test/templates.test.mjs test/completion.test.mjs test/completion-powershell.test.mjs — 36 passed, 3 platform-dependent skips
• PowerShell 5.1 TabExpansion2 functional coverage verifies template install, --dry-run, and the templates --into alias
• full corepack pnpm test — 707 passed and 193 skipped; the same 11 Windows environment failures were reproduced on clean upstream 053ec57, which passed 701 and skipped 193
• npm pack --dry-run — passed