Add automatic PR labeling workflow

[PAMulligan]

Description

Add a GitHub Actions workflow that automatically applies area labels to pull requests based on which files are changed.

Why

Consistent labeling helps maintainers triage PRs quickly and provides contributors with immediate feedback that their PR was categorized correctly. Manual labeling is error-prone and time-consuming.

Acceptance Criteria

• Create .github/labeler.yml with path-to-label mappings
• Create .github/workflows/labeler.yml using actions/labeler
• Map file paths to area labels:
  • scripts/** → area: scripts
  • .claude/agents/** → area: agents
  • .claude/skills/** → area: skills
  • .github/workflows/** → area: ci-cd
  • templates/** → area: templates
  • docs/** → area: docs
  • .claude/pipeline.config.json → area: pipeline
• Test with a sample PR
• CI passes

Example

.github/labeler.yml:

"area: scripts":
  - changed-files:
    - any-glob-to-any-file: 'scripts/**'

"area: agents":
  - changed-files:
    - any-glob-to-any-file: '.claude/agents/**'