feat: safely export custom configurations without overwriting files

[lonecoding]

Changes

Saving a custom configuration with shell redirection can truncate an existing file before the generator rejects an invalid group. Add --output PATH to create a new file exclusively after validation, supporting preset, custom-group and interactive modes. Existing files and symlink targets remain untouched, and prompts/status stay on stderr.

Interactive EOF previously exited successfully without producing a configuration. It now reports failure; blank Enter still selects the recommended preset and Ctrl+C cancels with exit status 130. Update the README, setup examples, maintenance notes and changelog to explain safe exports.

Source and testing

• Scope: local configuration generator only; no upstream rule or published preset changes.
• Verification date: 2026-09-12. Local Node.js 22.23.1 and Python 3.11.15.
• All 14 JavaScript tests pass, including all 32,768 policy-group selections, file preservation, symlinks, paths with spaces, invalid arguments and interactive EOF.
• All 28 Python tests, 34 offline routing cases and every generation/validation check in CONTRIBUTING.md pass.
• Manual terminal check: Ctrl+C exits 130, reports cancellation and creates no output file.
• git diff --check passes. No Quantumult X/iOS device testing: this change affects the desktop generator; app configuration contents are unchanged.

Checklist

• Checked generated compatibility output and READMEs; ran validation, routing cases, and unit tests in CONTRIBUTING.md.
• Recorded verification limits; no device testing was performed.
• Included no servers, subscriptions, keys, or personal information.
• Preserved existing policies and updated generator documentation together.