close-older-issues closes issues from different calling workflows

[strawgate]

Problem

Explore: Live Elasticsearch closed elastic/ai-github-actions-playground#1138, which belongs to medium-ideas-man.

When multiple workflows call the same reusable workflow via workflow_call, close-older-issues can't distinguish between them:

1. Shared workflow ID. GH_AW_WORKFLOW_ID is derived from the reusable workflow's filename at compile time, not the caller's. All callers of gh-aw-internal-gemini-cli.lock.yml share the same ID and close each other's issues.

2. Search substring matching. GitHub search for "gh-aw-workflow-id: gh-aw-internal-gemini-cli" also matches issues containing gh-aw-internal-gemini-cli-web-search, crossing even different reusable workflows. Adding --> to the query doesn't help — GitHub tokenizes on HTML comment delimiters.

Suggested fixes

Workflow ID: Incorporate the calling workflow's identity into the marker. Within a workflow_call, ${{ github.repository }}/${{ github.workflow }} would uniquely identify the caller across repos. Since GH_AW_WORKFLOW_ID is also used for branch names, this likely needs a separate env var for the marker.

Search precision: GitHub search can't exact-match these markers, so add a post-search filter on the issue body — the same pattern findCommentsWithTrackerId already uses:

const exactMarker = `<!-- gh-aw-workflow-id: ${workflowId} -->`;
const filtered = results.filter(item => item.body?.includes(exactMarker));