feat: auto-update CLI on major version gap

[ali-wljray]

Summary

Add auto-update logic that automatically upgrades the CLI when a significant version gap is detected, instead of just showing a notification banner.

Auto-update trigger conditions

The CLI will auto-update when either condition is met:

1. Major version bump — the latest major version > current major version (e.g. 2.0.0 vs 1.x.x)
2. Large minor gap — same major version, but latest.minor - current.minor > 3 (e.g. 1.7.0 vs 1.3.0)

For smaller version differences, the existing "Update available" banner is preserved.

示例输出效果

大版本自动更新时：

  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
  ⚡ Major update detected: 1.2.3 → 2.0.0
  Auto-updating to keep your CLI up to date...
  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✓ Update complete: 1.2.3 → 2.0.0
  Run bl --version to verify.

Changes

packages/cli/src/utils/update-checker.ts

• Added isMajorUpgrade(latest, current) — determines if the version gap warrants auto-update
• Added performAutoUpdate(currentVersion, latestVersion) — executes the auto-update flow:
  • Friendly visual banner distinguishing "Major update" vs "Significant update"
  • Runs npm install -g bailian-cli@latest
  • Verifies the installed version
  • Updates local cache (update-state.json)
  • Syncs agent skill (npx skills add modelstudioai/cli --all -g -y)
  • Graceful fallback on failure with manual update instructions

packages/cli/src/main.ts

• Post-command update notification now branches:
  • Major upgrade → calls performAutoUpdate() for automatic upgrade
  • Minor upgrade → shows the existing notification banner

Design decisions

• Auto-update runs after the user's command completes, so it never blocks the current operation
• All errors are silently handled — auto-update failure never crashes the CLI
• CI / non-TTY environments are skipped (inherits existing checkForUpdate guard)
• The update banner uses distinct messaging for major vs significant updates