Revise release notes agent for Olive repository

[xiaoyu-work]

Updated the release notes agent to generate structured Olive release notes by comparing the latest release branch with the current main branch, summarizing user-facing changes, and formatting them into release-note sections.

Describe your changes

Checklist before requesting a review

• Add unit tests for this change.
• Make sure all tests can pass.
• Update documents if necessary.
• Lint and apply fixes to your code by running lintrunner -a
• Is this a user-facing change? If yes, give a description of this change to be included in the release notes.

(Optional) Issue link

[Copilot]

Pull request overview

Adds a GitHub Copilot “release notes agent” definition for the Olive repository, intended to generate structured, user-facing release notes by diffing the latest release branch against main and formatting the results into standardized sections.

Changes:

• Introduced a new release-notes agent prompt with an explicit workflow for selecting the prior release baseline and collecting PR/commit metadata.
• Defined a fixed release-notes output structure (sections, bullet format rules, and classification guidance).
• Added quality requirements and a sample “Example style” release notes block to guide tone and content.