feat: improve git upgrade handlers

[saberzero1]

• Significantly improved Quartz upgrade reliability with smart conflict resolution.
  • Upgrades now automatically succeed when the user has only modified dedicated user files (quartz.config.yaml, quartz.lock.json, quartz.ts, quartz/styles/custom.scss, content/, .github/, quartz/static/, and quartz/styles/syncer/).
  • Framework files (everything else) automatically accept upstream changes during upgrade.
  • User-owned files are preserved across upgrades via snapshot and restore, even if upstream cleanly modified them.
  • If the user has modified framework files, the upgrade fails with a specific list of which files were modified, instead of a generic merge conflict error.
  • quartz.config.default.yaml is automatically accepted from upstream without blocking the upgrade.
• Removed unnecessary post-merge commits when user files were not changed by upstream.
• Updated upgrade notification to reference the in-app Upgrade button instead of npx quartz upgrade.
  • npx quartz upgrade is now only recommended as a fallback when the in-app upgrade fails.