[changelog] Add changelog for java/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 `java/v1.0.9` entry to `CHANGELOG.md` with 2 highlighted features and 9 other changes spanning Java-specific improvements and cross-SDK updates that landed in this release.

**Highlighted features:**
- `userPromptTransformed` hook via `SessionHooks.setOnUserPromptTransformed`
- Custom JSON schema override for `@CopilotToolParam` + compile-time blank-description enforcement

**Other changes (9):** disabled MCP servers, `agentStop` hook, `GitHubMcpToolConfig`, managed approval on permissions, `enableExperimentalMode`, additional directories, `customAgentsLocalOnly` forwarding, `ReasoningEffort.max`, and version-script bugfix.

> Generated by [Release Changelog Generator](https://github.com/github/copilot-sdk/actions/runs/31060734155) · sonnet46 70.5 AIC · ⌖ 9.03 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/java-v1.0.9-0adf9c759a4da870': 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/31060734155)
>
> 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 31060734155 -n agent -D /tmp/agent-31060734155

# Fetch the bundle into a temporary ref, then update the local branch
git fetch /tmp/agent-31060734155/aw-changelog-java-v1.0.9.bundle refs/heads/changelog/java-v1.0.9:refs/bundles/create-pr-changelog-java-v1-0-9-0adf9c759a4da870-3276e03d
git update-ref refs/heads/changelog/java-v1.0.9-0adf9c759a4da870 refs/bundles/create-pr-changelog-java-v1-0-9-0adf9c759a4da870-3276e03d
git checkout changelog/java-v1.0.9-0adf9c759a4da870
# 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-v1-0-9-0adf9c759a4da870-3276e03d

# Push the branch to origin
git push origin changelog/java-v1.0.9-0adf9c759a4da870

# Create the pull request
gh pr create --title '[changelog] Add changelog for java/v1.0.9' --base main --head changelog/java-v1.0.9-0adf9c759a4da870 --repo github/copilot-sdk
```

Review the generated changelog details and the protected-file notice in the issue, then download workflow run 31060734155's agent artifact. Follow the listed git commands to restore the changelog branch and create the pull request against main. Done means CHANGELOG.md contains the java/v1.0.9 entry and the proposed changes are submitted for review.

Written by the indexing model from the issue text.