Centralize auto-label mappings into a config file

[Devansh-567]

The label mappings in .github/workflows/auto-label.yml are hardcoded as inline JavaScript. This has a few problems:

• Contributors who want to add a label keyword must edit workflow logic, which is harder to review and requires understanding the JS
• The issue and PR mapping objects are duplicated separately with subtle inconsistencies (e.g. [Tofino] vs [tofino])
• Several prefixes used in real PRs ([ebpf], [ubpf], [graphs]) have no mapping at all
  I'd like to move the mappings to .github/label-mappings.json and have the workflow read it at runtime. Would the maintainers be open to this approach?