[changelog] Add changelog for v1.0.9

Start with CHANGELOG.md and the linked workflow run 31060763432; download the agent artifact and inspect the generated v1.0.9 change before following the provided git and gh commands. Done means the v1.0.9 entry is proposed in a pull request and the protected-file review requirement is satisfied.

[!CAUTION]
Protected files were modified in this change.
This pull request is in request_review mode and requires explicit human scrutiny before merge.

Protected files: CHANGELOG.md

Adds the CHANGELOG.md entry for the v1.0.9 stable release.

This release includes 5 highlighted features and 12 other changes, with 14 new contributors.

Generated by Release Changelog Generator · sonnet46 43.5 AIC · ⌖ 9.44 AIC · ⊞ 8.6K · ◷

[!NOTE]
This was originally intended as a pull request, but the git push operation failed.

Original error: pushSignedCommits: refusing unsigned push for branch 'changelog/v1.0.9-76b5be63a6cc02d0': Signed-commit payload violates file-protection policy (request_review): CHANGELOG.md

Workflow Run: View run details and download bundle artifact

The bundle file is available in the agent artifact in the workflow run linked above.

To create a pull request with the changes:

# Download the artifact from the workflow run
gh run download 31060763432 -n agent -D /tmp/agent-31060763432

# Fetch the bundle into a temporary ref, then update the local branch
git fetch /tmp/agent-31060763432/aw-changelog-v1.0.9.bundle refs/heads/changelog/v1.0.9:refs/bundles/create-pr-changelog-v1-0-9-76b5be63a6cc02d0-ce26e761
git update-ref refs/heads/changelog/v1.0.9-76b5be63a6cc02d0 refs/bundles/create-pr-changelog-v1-0-9-76b5be63a6cc02d0-ce26e761
git checkout changelog/v1.0.9-76b5be63a6cc02d0
# Ensure the working tree matches the updated branch
git reset --hard
# Remove the temporary bundle ref
git update-ref -d refs/bundles/create-pr-changelog-v1-0-9-76b5be63a6cc02d0-ce26e761

# Push the branch to origin
git push origin changelog/v1.0.9-76b5be63a6cc02d0

# Create the pull request
gh pr create --title '[changelog] Add changelog for v1.0.9' --base main --head changelog/v1.0.9-76b5be63a6cc02d0 --repo github/copilot-sdk