Feature: lint PR titles

### Problem Statement

Sometimes community users forget about adjusting the PR title (and merge commit) to the style guide of the Git repository. `aicommit` already uses the GPT model to adjust wording of particular commits, but not the PR title.

### Proposal

Let's introduce `aicommit lint` command which can validate the proposed PR title. The command can be executed as [Github Action](https://github.com/features/actions) and fail CI job if the PR does not meet style guide requirements.

**Proposed Workflow**

Command: `aicommit lint --pr-title "fix: set dogfood go version to match go.mod"`

1. Load the style guide rules.
2. Prepare the prompt: _Ask to lint the PR title according to the style guide. Expected result: OK or suggestion._
3. Exit status 0 or output the suggestion.