[changelog] Add changelog for v1.0.8

> [!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 changelog entry for v1.0.8 with 1 highlighted feature (per-agent reasoning effort) and 2 other changes.

> [!WARNING]
>
> Firewall blocked 1 domain
>
> The following domain was blocked by the firewall during workflow execution:
>
> - `awmgmcpg`
>> To allow these domains, add them to the `network.allowed` list in your workflow frontmatter:
>
> ```yaml
> network:
> allowed:
> - defaults
> - "awmgmcpg"
> ```
>
> See [Network Configuration](https://github.github.com/gh-aw/reference/network/) for more information.
>
>

> Generated by [Release Changelog Generator](https://github.com/github/copilot-sdk/actions/runs/29965645125) · 39.1 AIC · ⌖ 5.54 AIC · ⊞ 7.2K · [◷](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/v1.0.8-27a80650ba156db9': 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/29965645125)
>
> 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 29965645125 -n agent -D /tmp/agent-29965645125

# Fetch the bundle into a temporary ref, then update the local branch
git fetch /tmp/agent-29965645125/aw-changelog-v1.0.8.bundle refs/heads/changelog/v1.0.8:refs/bundles/create-pr-changelog-v1-0-8-27a80650ba156db9-49953e31
git update-ref refs/heads/changelog/v1.0.8-27a80650ba156db9 refs/bundles/create-pr-changelog-v1-0-8-27a80650ba156db9-49953e31
git checkout changelog/v1.0.8-27a80650ba156db9
# 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-v1-0-8-27a80650ba156db9-49953e31

# Push the branch to origin
git push origin changelog/v1.0.8-27a80650ba156db9

# Create the pull request
gh pr create --title '[changelog] Add changelog for v1.0.8' --base main --head changelog/v1.0.8-27a80650ba156db9 --repo github/copilot-sdk
```

Start with CHANGELOG.md and the linked Release Changelog Generator workflow run. Download the agent artifact using the provided gh commands, inspect the generated v1.0.8 entry, then create the pull request against main. Done means the changelog contains the highlighted per-agent reasoning effort feature and two other changes and receives the required human review.

索引モデルが issue の本文から書いたものです。