fix: restore deep-research compatibility workflow

> [!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: `.github/workflows/gh-aw-deep-research.lock.yml`

Restores the documented legacy `gh-aw-deep-research.lock.yml` reusable workflow for downstream consumers and adds its mapping to `scripts/backwards-compat.sh` so the copy remains reproducible.

Closes \#1565

Validation: `make lint`; `UV_CACHE_DIR=/tmp/gh-aw/agent/uv-cache make test` (54 passed).

---
[What is this?](https://ela.st/github-ai-tools) | [From workflow: Trigger Bug Hunter](https://github.com/elastic/ai-github-actions/actions/runs/29329375142)

Give us feedback! React with 🚀 if perfect, 👍 if helpful, 👎 if not.

---

> [!NOTE]
> This was originally intended as a pull request, but the git push operation failed.
>
> **Workflow Run:** [View run details and download bundle artifact](https://github.com/elastic/ai-github-actions/actions/runs/29329375142)
>
> 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 29329375142 -n agent -D /tmp/agent-29329375142

# Fetch the bundle into a temporary ref, then update the local branch
git fetch /tmp/agent-29329375142/aw-fix-restore-deep-research-compat.bundle refs/heads/fix/restore-deep-research-compat:refs/bundles/create-pr-fix-restore-deep-research-compat-7d91258fd078318b-61469beb
git update-ref refs/heads/fix/restore-deep-research-compat-7d91258fd078318b refs/bundles/create-pr-fix-restore-deep-research-compat-7d91258fd078318b-61469beb
git checkout fix/restore-deep-research-compat-7d91258fd078318b
# Ensure the working tree matches the updated branch
git reset --hard
# Remove the temporary bundle ref
git update-ref -d refs/bundles/create-pr-fix-restore-deep-research-compat-7d91258fd078318b-61469beb

# Push the branch to origin
git push origin fix/restore-deep-research-compat-7d91258fd078318b

# Create the pull request
gh pr create --title 'fix: restore deep-research compatibility workflow' --base main --head fix/restore-deep-research-compat-7d91258fd078318b --repo elastic/ai-github-actions
```

Review .github/workflows/gh-aw-deep-research.lock.yml and scripts/backwards-compat.sh first, then run the listed validation commands: make lint and UV_CACHE_DIR=/tmp/gh-aw/agent/uv-cache make test. Done means the documented reusable workflow is restored, its mapping remains reproducible, and both validations pass.

Written by the indexing model from the issue text.