[Documentation] Define TestPlatform integration contract

[ConnorQi01]

Summary

Define iOS-Automation-Framework as the first TestPlatform integration sample and keep its responsibilities focused on automation test code, suite metadata, and deterministic report output.

Changes

• Add test-platform.yml with API, iOS UI, and performance suite definitions.
• Add docs/platform-integration.md to clarify boundaries between TestPlatform, Local Agent, app artifacts, and this test repository.
• Update README positioning so tools/webui is described as a local demo console, not the platform center.
• Ignore local Web UI run outputs and temporary Playwright probe scripts.

Test Plan

• Parsed test-platform.yml with Python yaml.safe_load.
• No full test run was executed because this change only updates documentation, ignore rules, and suite metadata.

Closes #23