feat: detect old onkernel/tap and show migration instructions

[masnwilliams]

Summary

• Detects when users have kernel installed from the old onkernel/tap instead of kernel/tap
• Shows migration instructions instead of the standard upgrade command when on the old tap

This helps users who installed via onkernel/tap/kernel migrate to kernel/tap/kernel which is where new releases are published.

Example output

When a user on onkernel/tap sees an update available:

 INFO  A new release of kernel is available: 0.13.5 → 0.14.0
 INFO  Release notes: https://github.com/kernel/cli/releases/tag/v0.14.0

 WARNING  You have kernel installed from the old tap (onkernel/tap).
 WARNING  To upgrade, switch to the new tap:

  brew uninstall kernel
  brew install kernel/tap/kernel

Test plan

• Verify detection works on a machine with onkernel/tap/kernel installed
• Verify normal upgrade message shows for kernel/tap/kernel users
• Verify non-Homebrew installs still show normal upgrade message

---

Note

Adds logic to identify Homebrew installs from the deprecated onkernel/tap by scanning INSTALL_RECEIPT.json in common Cellar paths.

• New isOnOldBrewTap() checks for "tap":"onkernel/tap" in Homebrew receipts
• printUpgradeMessage now warns and shows brew uninstall kernel + brew install kernel/tap/kernel when on old tap; otherwise retains existing upgrade suggestion behavior

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JSON string matching may miss whitespace variations

Medium Severity

The isOnOldBrewTap() function uses literal string matching "tap":"onkernel/tap" (no space after colon) to detect the tap source. However, JSON formatting can vary - Homebrew may output "tap": "onkernel/tap" (with space after colon) depending on whether it uses compact or pretty-printed JSON. If the actual file has whitespace differences, the detection silently fails, and users on the old tap receive incorrect upgrade instructions (brew upgrade kernel/tap/kernel) instead of migration instructions. Since the encoding/json package is already imported, parsing the JSON properly would be more reliable.