Make mention-in-pr timeout configurable

> [!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: `README.md`, `.github/workflows/gh-aw-mention-in-pr.lock.yml`, `.github/workflows/gh-aw-mention-in-pr.md`

Closes \#1686

Adds a configurable `timeout-minutes` workflow-call input to the standard `mention-in-pr` workflow, defaulting to 30 minutes and wiring it through the generated lock file. Documents the override in the consumer example and README.

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

> [!WARNING]
>
> Firewall blocked 2 domains
>
> The following domains were blocked by the firewall during workflow execution:
>
> - `awmgmcpg`
> - `models.dev`
>> To allow these domains, add them to the `network.allowed` list in your workflow frontmatter:
>
> ```yaml
> network:
> allowed:
> - defaults
> - "awmgmcpg"
> - "models.dev"
> ```
>
> See [Network Configuration](https://github.github.com/gh-aw/reference/network/) for more information.
>
>

---
[What is this?](https://ela.st/github-ai-tools) | [From workflow: Trigger Text Auditor](https://github.com/elastic/ai-github-actions/actions/runs/30272073629)

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---

> [!NOTE]
> This was originally intended as a pull request, but the git push operation failed.
>
> **Original error:** pushSignedCommits: refusing unsigned push for branch 'fix/mention-in-pr-timeout-270ebc501f1235b8': Signed-commit payload violates file-protection policy (request_review): gh-agent-workflows/mention-in-pr/README.md, .github/workflows/gh-aw-mention-in-pr.lock.yml, .github/workflows/gh-aw-mention-in-pr.md
>
> **Workflow Run:** [View run details and download bundle artifact](https://github.com/elastic/ai-github-actions/actions/runs/30272073629)
>
> 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 30272073629 -n agent -D /tmp/agent-30272073629

# Fetch the bundle into a temporary ref, then update the local branch
git fetch /tmp/agent-30272073629/aw-elastic-ai-github-actions-fix-mention-in-pr-timeout.bundle refs/heads/fix/mention-in-pr-timeout:refs/bundles/create-pr-fix-mention-in-pr-timeout-270ebc501f1235b8-50ff06c1
git update-ref refs/heads/fix/mention-in-pr-timeout-270ebc501f1235b8 refs/bundles/create-pr-fix-mention-in-pr-timeout-270ebc501f1235b8-50ff06c1
git checkout fix/mention-in-pr-timeout-270ebc501f1235b8
# 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-mention-in-pr-timeout-270ebc501f1235b8-50ff06c1

# Push the branch to origin
git push origin fix/mention-in-pr-timeout-270ebc501f1235b8

# Create the pull request
gh pr create --title 'Make mention-in-pr timeout configurable' --base main --head fix/mention-in-pr-timeout-270ebc501f1235b8 --repo elastic/ai-github-actions
```

Inspect the standard mention-in-pr workflow, .github/workflows/gh-aw-mention-in-pr.lock.yml, .github/workflows/gh-aw-mention-in-pr.md, and the README examples. Confirm how workflow-call inputs reach the generated lock file, then run make compile, make lint, and the documented test command. Done means timeout-minutes defaults to 30, can be overridden, and the consumer documentation explains it.

Written by the indexing model from the issue text.