[Feat] Machine-friendly exit codes for error classification

[samzong]

What's changed?

• Add internal/exitcode package defining typed exit codes: NoStagedChanges=10, NotGitRepo=11, LLMError=12
• main.go extracts exitcode.Error via errors.As before falling back to os.Exit(1)
• Sentinel errors git.ErrNotGitRepo and llm.ErrLLM enable errors.Is classification
• Shared classifyError() in cmd package eliminates duplication between root and tag commands
• Unit tests cover all three exit code paths (10, 11, 12)

Why

• All errors currently exit 1, forcing agents/CI to parse stderr text to distinguish error categories
• Structured exit codes let automation retry transient LLM failures (12) vs report user errors (10, 11) without text parsing

Closes #55

[gemini-code-assist]

Code Review

This pull request introduces a centralized exit code management system by adding an internal exitcode package and updating error handling in the git, llm, and cmd packages. The review feedback identifies a potential compilation issue on Go versions earlier than 1.20 due to the use of multiple error-wrapping verbs and suggests wrapping unclassified errors in the tag command with userFacingError to maintain consistent formatting across the CLI.

[gemini-code-assist]

The use of multiple %w format verbs for error wrapping was introduced in Go 1.20. If this project needs to maintain compatibility with older Go versions, this will cause a compilation error.

For compatibility with Go versions < 1.20, you could wrap only ErrLLM and include the original error's text in the message. This preserves the ability to check for ErrLLM while keeping the context.

For example:

return "", fmt.Errorf("failed to call LLM: %v: %w", err, ErrLLM)

This would mean errors.As would no longer work for the original err, but errors.Is for ErrLLM would still function correctly.

[gemini-code-assist]

Similar to the comment on line 104, the use of multiple %w verbs for error wrapping requires Go 1.20+. If compatibility with older versions is needed, consider an alternative wrapping strategy.

For example:

return "", "", fmt.Errorf("failed to call LLM: %v: %w", err, ErrLLM)

[gemini-code-assist]

To ensure consistent error message formatting across all commands, unclassified errors from the tag command should also be wrapped in userFacingError, similar to how the root command handles them. This will provide a uniform experience for users, with errors prefixed by gmc: .

After making this change, please also ensure all error return paths in runTagCommand use this wrapper for consistency (e.g., the one at line 70 seems to have been missed).

Suggested change

	return err
	return userFacingError{msg: fmt.Sprintf("gmc: %v", err), err: err}