Improve changelog agent output formatting

[SteveSandersonMS]

Follow-up improvements to the release changelog agent based on initial test run:

• Don't include version heading in GitHub Release notes — the release already has a title showing the version, so the ## [vX.Y.Z] heading was redundant
• Omit ### Other changes subheading when there are no highlighted features above it — just list bullets directly under the version heading
• Add ### New contributors section listing first-time contributors with a link to their first PR, matching the useful info GitHub's auto-generated notes already provided

[Copilot]

Pull request overview

This PR improves the output formatting of the release changelog agentic workflow (release-changelog.md) by making three targeted improvements: removing the redundant version heading from GitHub Release notes, conditionally omitting the ### Other changes subheading when there are no highlighted features, and adding a ### New contributors section for first-time contributors.

Changes:

• Adds instruction for identifying new contributors in Step 3 and formatting them in Step 4
• Updates Step 6 to omit the version heading from GitHub Release notes
• Extends the example output to demonstrate the new ### New contributors section

Comments suppressed due to low confidence (2)

.github/workflows/release-changelog.md:152

• The "Key rules visible in the example" summary (lines 148–152) has been updated with a new contributors section in the example output but the summary doesn't mention the new ### New contributors rule. The AI agent uses these key rules as a quick reference, so the omission means it may not consistently apply the new contributors section. A bullet should be added here, e.g. "A ### New contributors section lists first-time contributors with a link to their first PR; omit it if there are none."

This issue also appears on line 101 of the same file.

**Key rules visible in the example:**
- Highlighted features get their own `### Feature:` heading, a short description, and code snippets
- Code snippets are TypeScript and C# primarily; Go/Python only when listing one-liner equivalents or when meaningfully different
- The `### Other changes` section is a flat bulleted list with lowercase `bugfix:` / `feature:` / `improvement:` prefixes
- PR numbers are linked inline, not at the end with author attribution (keep it clean)

.github/workflows/release-changelog.md:101

• Step 6 instructs the agent to start the GitHub Release notes "directly with the feature sections or other changes list," but does not clarify whether the ### New contributors section should also be included in the release notes. Based on the PR description ("matching the useful info GitHub's auto-generated notes already provided"), it seems intended to appear there too. The instruction should explicitly state whether to include it, to avoid ambiguity for the AI agent.

Use the `update-release` output to replace the auto-generated release notes with your nicely formatted changelog. **Do not include the version heading** (`## [vX.Y.Z](...) (date)`) in the release notes — the release already has a title showing the version. Start directly with the feature sections or other changes list.