feat: support pull_request.labeled action for track_progress

[0x-JP]

Summary

• Adds "labeled" to the supported PR actions in both detectMode() and validateTrackProgressEvent() in src/modes/detector.ts
• Enables label-gated PR reviews with track_progress enabled
• Adds test for pull_request.labeled with track_progress: true

Context

Fixes #418

Currently, using track_progress: "true" with pull_request: types: [labeled] throws:

track_progress for pull_request events is only supported for actions: opened, synchronize, ready_for_review, reopened. Current action: labeled

The downstream code (tag mode, prompt construction, branch setup, comment creation) is entirely action-agnostic for PR events, so the restriction was unnecessarily narrow. This change adds "labeled" to both validation arrays, enabling label-gated workflows like:

on:
  pull_request:
    types: [labeled]

jobs:
  review:
    if: github.event.label.name == 'my-review-label'
    steps:
      - uses: anthropics/claude-code-action@v1
        with:
          prompt: "Review this PR"
          track_progress: "true"

Test plan

• Unit test added for pull_request.labeled with track_progress: true
• All existing tests pass (bun test)
• TypeScript typecheck passes (bun run typecheck)
• Formatting passes (bun run format:check)

🤖 Generated with Claude Code