[changelog] Add changelog for v1.0.9

> [!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](https://github.com/github/copilot-sdk/actions/runs/31060763432) · sonnet46 43.5 AIC · ⌖ 9.44 AIC · ⊞ 8.6K · [◷](https://github.com/search?q=repo%3Agithub%2Fcopilot-sdk+%22gh-aw-workflow-id%3A+release-changelog%22&type=pullrequests)

---

> [!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](https://github.com/github/copilot-sdk/actions/runs/31060763432)
>
> The bundle file is available in the `agent` artifact in the workflow run linked above.

To create a pull request with the changes:

```sh
# 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
```

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.

索引モデルが issue の本文から書いたものです。