Add support for inline diff-level PR comments

[tarmojussila]

Feature: Inline Diff-Level PR Comments

Problem

Currently, the action posts review feedback as a single summary comment on the Pull Request.

While this works, it does not fully integrate with GitHub's native code review experience. Developers expect comments to appear directly on the relevant file and line in the diff.

Proposed Solution

Add support for posting inline review comments attached to specific files and lines in the Pull Request diff.

Instead of only posting a summary comment, the action should:

• Map model feedback to specific files
• Map feedback to exact line numbers when possible
• Use the GitHub Reviews API to create inline comments