feat: auto-assign reviewer when /request-review is used with no arguments

[juliethecao]

Feature Summary

The existing /request-review @user [@user ...] command from #5166 and #5222 requires the PR author to already know who to ping. For contributors unfamiliar with the codebase (e.g., first-time contributors, cross-team PRs), identifying the right reviewer is not obvious and often leads to review requests going to the wrong person or being left unassigned.

Command	What it does
/request-review @user [@user ...]	Request reviews from those users
/request-review (no args)	Auto-detects reviewer via git blame on changed files

Proposed Solution or Design

• Extend the /request-review comment command in .github/workflows/comment-commands.yml so that when called with no @mention arguments, it automatically identifies and requests a review from the person who most recently touched each file changed in the PR, which is determined via git blame.
• This makes /request-review a zero-configuration entry point: the author signals intent, and the system routes to the right person.

Affected Area

Deployment / Infrastructure

[juliethecao]

/take

[Ma77Ball]

@juliethecao, this seems like a really good feature to add. It will be very helpful for the community and for committers to manage this project.