feat: add main review guidelines library

[menny]

• Create reviewer_prompts/ directory with 7 specialized review philosophies.
• Refactor main guidelines resolution logic into testable helper.
• Update core/prompts/reviewer_prompt.md to be fully generic and behavior-driven.
• Set 'general' (Robust Maintainer) as the default guideline for the action and CLI.
• Ensure format and linting issues are skipped by the AI reviewer.
• Update README.md and DESIGN.md with new guideline library documentation.

Closes #27

[github-actions]

[Maintenance] [cmd/ai_reviewer/main.go:187-194] This error handling masks the underlying os.ReadFile error. If a user provides a valid file path that cannot be read (e.g., due to permissions, or if it is a directory), this silently falls back to GetLibraryPrompt and ultimately fails with a confusing "prompt not found" error. We should check if the error is os.ErrNotExist (or use errors.Is(err, fs.ErrNotExist)) before falling back, so we don't swallow actionable filesystem errors.

[Observation] [action.yml:101-105] Since the action falls back to checking if a local file exists with the name of the provided guidelines, if a repository accidentally contains a file literally named general (or google, minimalist) in its root, the action will pick that file up instead of using the built-in library prompt. This is an edge case, but worth noting for future debugging.

[Observation] [cmd/ai_reviewer/main.go:88] log.Fatalf is used here instead of the stderr logger instantiated earlier in the file (stderr := log.New(os.Stderr, "", 0)). While log.Fatalf still outputs to os.Stderr, it will use the standard Go log formatting (with timestamps), which differs from the rest of the diagnostic output. Using stderr.Printf followed by `os.Exit