Add structured native app-control actions to BasicAction

[NakaokaRei]

Summary

• add openURL, activateApp, quitApp, and getFrontmostApp to BasicAction
• expose the actions through both OpenAI backends, including prompts, parsing, history descriptions, and strict JSON schema
• implement Tier 1 app control with native macOS APIs instead of interpolated AppleScript:
  • NSWorkspace.open for URLs
  • NSRunningApplication.activate for activation
  • NSRunningApplication.terminate for quitting
  • NSWorkspace.frontmostApplication for frontmost-app lookup
• move generic Action.executeAppleScript execution onto a dedicated serial queue to avoid blocking the main actor
• add a repository-local Claude/Codex skill with manual sagui agent smoke-test commands

This incorporates the implementation concerns raised in the issue discussion, while retaining the structured AI action interface proposed by the issue.

Validation

• swift test (131 tests passed)
• swift package generate-documentation
• live sagui agent checks for getFrontmostApp and activateApp
• verified Calculator became the frontmost app after native activation

Closes #82

[github-actions]

🧪 Test Results

✅ All Tests Passed!

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