Add support for push events in agent mode

[robtaylor]

Summary

This PR adds support for push events in agent/automation mode, enabling CI integration testing of skills without requiring manual workflow_dispatch triggers.

Changes

• Import PushEvent type from @octokit/webhooks-types
• Add "push" to AUTOMATION_EVENT_NAMES constant
• Add PushEvent to AutomationContext payload union type
• Add case handler for "push" events in parseGitHubContext

Use Case

This enables reusable CI workflows (like skill testing) to run on push events to main branches, not just on PRs or manual triggers. For example:

on:
  push:
    branches: [main]
  pull_request:
    branches: [main]

jobs:
  test:
    uses: anthropics/skills/.github/workflows/skill-ci.yml@main
    with:
      run_claude_code_test: true
    secrets: inherit

Without this change, the action throws Error: Unsupported event type: push.

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