feat: extend review approval for 'comment' when uncertain

[menny]

This PR implements #32, introducing a third state to the review verdict: 💬 Comment.

Key Changes:

• Default Evaluation Prompt: Added core/prompts/approval_evaluation_prompt.md which defines the criteria for Approve, Reject, and Comment. It prioritizes approval velocity while allowing for a neutral Comment when the reviewer is uncertain.
• Embedded Guidelines: The evaluation guidelines are now embedded and injected into the main ReAct system prompt.
• Extraction Logic: Updated core/prompts/extraction_prompt.md to correctly map the 💬 Comment verdict to a structured COMMENT action with approved: false.
• Customizable Prompt: Added --approval-evaluation-prompt-file CLI flag and a corresponding GitHub Action input to allow overriding the default guidelines.
• Documentation: Updated README.md and DESIGN.md to reflect the new configuration options.

Tested locally with Bazel; all tests passed.

[github-actions]

The changes cleanly implement the new Comment state and correctly propagate the approval-evaluation-prompt-file through the CLI, Action inputs, and internal prompts. Environment variables are used appropriately in action.yml to prevent command injection, adhering to the project's security guidelines. Error handling and testing additions are also solid. Great work maintaining explicit handling of inputs and thorough test coverage.

[github-actions]

The changes cleanly implement the new Comment state, correctly propagate the prompt file, securely use environment variables, and have solid error handling and testing.