[changelog] Add changelog for java/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.md entry for the java/v1.0.8 release (2 user-facing changes: per-agent reasoning effort and strongly-typed expAssignments).

> [!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/29965619881) · 44.4 AIC · ⌖ 5.53 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/java-v1.0.8-e1e3ac881ac8d09a': 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/29965619881)
>
> 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 29965619881 -n agent -D /tmp/agent-29965619881

# Fetch the bundle into a temporary ref, then update the local branch
git fetch /tmp/agent-29965619881/aw-changelog-java-v1.0.8.bundle refs/heads/changelog/java-v1.0.8:refs/bundles/create-pr-changelog-java-v1-0-8-e1e3ac881ac8d09a-acc663cb
git update-ref refs/heads/changelog/java-v1.0.8-e1e3ac881ac8d09a refs/bundles/create-pr-changelog-java-v1-0-8-e1e3ac881ac8d09a-acc663cb
git checkout changelog/java-v1.0.8-e1e3ac881ac8d09a
# 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-java-v1-0-8-e1e3ac881ac8d09a-acc663cb

# Push the branch to origin
git push origin changelog/java-v1.0.8-e1e3ac881ac8d09a

# Create the pull request
gh pr create --title '[changelog] Add changelog for java/v1.0.8' --base main --head changelog/java-v1.0.8-e1e3ac881ac8d09a --repo github/copilot-sdk
```

从 CHANGELOG.md 和链接的 Release Changelog Generator 工作流运行记录开始。下载 `agent` 构件并检查 `aw-changelog-java-v1.0.8.bundle`，然后再应用或重新创建分支。完成的标准是，CHANGELOG.md 包含涵盖每个 agent 的推理工作量和强类型 expAssignments 的 java/v1.0.8 条目，随后提交所请求的 pull request。

由索引模型根据 Issue 内容生成。