Prevent self upgrade on Homebrew installs

[kindermax]

Summary

• Block lets self upgrade when the running binary is Homebrew-managed
• Reuse the existing Homebrew path detection already used by update notifications
• Add a unit test covering the Cellar install path and update the changelog

Testing

• go test ./internal/upgrade
• go test ./...
• lets test-bats

Summary by Sourcery

Block self-upgrade for Homebrew-managed installations and document the change.

Bug Fixes:

• Prevent lets self upgrade from overwriting Homebrew-managed Homebrew Cellar installs.

Documentation:

• Update changelog to note the fix for Homebrew-managed self-upgrades.

Tests:

• Add unit test ensuring Homebrew-managed binaries are not self-upgraded and no download occurs.

[sourcery-ai]

Reviewer's Guide

Prevents lets self upgrade from modifying Homebrew-managed binaries by detecting Homebrew Cellar install paths before upgrade, returning a clear error instructing users to use brew upgrade, while adding a regression test and updating the changelog.

Updated class diagram for BinaryUpgrader Homebrew detection

classDiagram
  class RepoRegistry {
    <<interface>>
    +GetLatestRelease(ctx Context) string
    +DownloadBinary(ctx Context, version string) BinaryData
  }

  class BinaryUpgrader {
    -RepoRegistry registry
    -string currentVersion
    -string binaryPath
    +BinaryUpgrader(reg RepoRegistry, currentVersion string) BinaryUpgrader
    +Upgrade(ctx Context) error
  }

  class HomebrewDetection {
    +isHomebrewInstall(binaryPath string) bool
  }

  BinaryUpgrader --> RepoRegistry : uses
  BinaryUpgrader --> HomebrewDetection : calls isHomebrewInstall

Loading

File-Level Changes

Change	Details	Files
Block self-upgrade when running from a Homebrew-managed Cellar install and surface a user-facing error that points to the correct brew upgrade command.	Short-circuit the upgrade flow if the current binary path is detected as a Homebrew install using existing Homebrew detection logic. Return a formatted error indicating Homebrew installs must be upgraded via brew upgrade lets-cli/tap/lets instead of performing a download or file replacement. Ensure the registry lookup and binary replacement logic is skipped for Homebrew installs.	internal/upgrade/upgrade.go
Add regression coverage for Homebrew Cellar installs to ensure they are not modified by self-upgrade and that no download occurs.	Introduce a new subtest that creates a fake Homebrew Cellar directory structure and writes a mock lets binary at the expected path. Instantiate BinaryUpgrader with the Homebrew-like binary path and assert that Upgrade returns an error containing the suggested brew upgrade command. Verify that the binary contents remain at the original version and that the expected download file does not exist after the attempted upgrade.	internal/upgrade/upgrade_test.go
Document the new Homebrew self-upgrade protection behavior.	Add an Unreleased changelog entry describing that lets self upgrade no longer overwrites Homebrew-managed installs and references the associated issue.	docs/docs/changelog.md

Possibly linked issues

• Update upgrade command logic to match autoupdate logic #338: The PR adds Homebrew install detection to self upgrade, blocking overwrites and documenting the fix for this issue.

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[sourcery-ai]

Hey - I've left some high level feedback:

• Consider extracting the brew upgrade lets-cli/tap/lets string into a shared constant so it’s not duplicated between the implementation and the test and is easier to change if the tap name ever changes.
• If callers might need to handle the Homebrew case specially, you could introduce a sentinel error (e.g., ErrHomebrewInstall) instead of relying on matching the error message text.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider extracting the `brew upgrade lets-cli/tap/lets` string into a shared constant so it’s not duplicated between the implementation and the test and is easier to change if the tap name ever changes.
- If callers might need to handle the Homebrew case specially, you could introduce a sentinel error (e.g., `ErrHomebrewInstall`) instead of relying on matching the error message text.

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