[Build/CI] Re-enable actions/labeler with canonical schema-compatible config

[ashleyshaw]

Build/CI Summary

PR #418 temporarily disabled actions/labeler@v5 because .github/labeler.yml is not compatible with the action schema.
This follow-up restores deterministic branch/file auto-labelling while keeping canonical label governance in .github/labels.yml and issue types in .github/issue-types.yml.

Steps / Checklist

• Decide target architecture:
  • Option A: keep actions/labeler and make .github/labeler.yml fully schema-compatible.
  • Option B: retire actions/labeler and rely only on scripts/agents/labeling.agent.js.
• If Option A is chosen, split agent-only rules from action-compatible rules.
• Add validation for labeler.yml shape in CI (fail fast before workflow runtime).
• Update workflow comments and docs to reflect final source of truth and execution path.
• Verify branch-based and file-based labels on a test PR.
• Ensure no duplicate/conflicting label application between action and agent.

Acceptance Criteria

• Unified Labeling, Status, and Type Assignment passes on test PRs without manual intervention.
• No workflow step fails due to actions/labeler schema errors.
• Canonical labels remain sourced from .github/labels.yml only.
• Issue types remain sourced from .github/issue-types.yml only.
• Label assignment is deterministic for:
  • branch prefix (feat/, fix/, docs/, ci/, etc.)
  • changed paths (.github/workflows/**, docs, scripts, etc.)
• Documentation updated:
  • /Users/ash/Studio/LightSpeedWP.Agency/.github/docs/ISSUE_LABELS.md
  • /Users/ash/Studio/LightSpeedWP.Agency/.github/docs/ISSUE_TYPES.md
  • /Users/ash/Studio/LightSpeedWP.Agency/.github/docs/MIGRATION.md (if mappings/process changed)

Additional Context

• Temporary mitigation was merged under PR fix(labels): reconcile canonical label families #418 to unblock CI.
• This issue is the hardening follow-up and should be completed before future label-system expansion.

References

• PR fix(labels): reconcile canonical label families #418: CI fixes and temporary actions/labeler disablement
• /Users/ash/Studio/LightSpeedWP.Agency/.github/.github/workflows/labeling.yml
• /Users/ash/Studio/LightSpeedWP.Agency/.github/.github/labeler.yml
• /Users/ash/Studio/LightSpeedWP.Agency/.github/scripts/agents/labeling.agent.js

[linear-code]

LS-714