add `--hint` flag for contextual generation guidance

[rm-hull]

Problem

Currently, the LLM analyzes the entire diff provided. In large or complex diffs, it may assign equal weight to minor style changes, refactors, and critical bug fixes. This can lead to commit summaries that miss the main intent of the commit.

Proposed Solution

Add a --hint (or -H) flag to allow users to provide context to the LLM before the generation phase.

Usage Example

git commit-summary --hint "prioritize the authentication flow changes, ignore the minor CSS updates"

Implementation Notes

• The hint should be injected into the LLM prompt context to steer its analysis.
• The hint should be passed to the prompt template as a new variable.
• This differs from the existing -m flag, which is used for appending text to the final commit message after generation.