feat: add check-pr-issue-link reusable workflow

[glageju]

Partially closes https://github.com/stacklok/claude-plugins/issues/275

Summary

• Adds check-pr-issue-link.yml, copied from StacklokLabs/workflow-templates, as a reusable workflow (workflow_call) callable by repos in the StacklokLabs organization.
• On PR open/edit/reopen/sync, checks the PR body for a closing keyword (closes/fixes/resolves/etc.) referencing a valid issue number. Posts/updates a comment and applies a needs-issue-link label when the link is missing or invalid; clears both once a valid link is present.
• Pins actions/github-script to the same SHA (v7.1.0) already used by needs-triage.yml in this repo, consistent with existing convention.

Test plan

• Confirm the workflow YAML is valid (lint/parse check in CI)
• Enable "Accessible from repositories in the 'StacklokLabs' organization" under Settings → Actions → General → Access for this repo
• Call the workflow from a downstream repo with workflow_call and verify label/comment behavior on a PR with no issue link, an invalid issue link, and a valid issue link

Out of scope

• Checking the Issue link from cross org/ cross repo reference

[github-actions]

Missing Issue Link

This PR does not appear to reference a GitHub issue.
Please add a closing keyword, e.g.: Closes #123
Supported: closes, fixes, resolves, close, fix, resolve (case insensitive)