feat(skills): expand platform coverage and add multi-product integration guide (v1.2.0)

[digitallysavvy]

New platform reference files:

• rtc/react-native.md — react-native-agora engine init, events, RtcSurfaceView, cleanup
• rtc/flutter.md — agora_rtc_engine init, AgoraVideoView, RtcEngineEventHandler, cleanup
• rtm/ios.md — AgoraRtmClientKit v2 (Swift): init, login, subscribe, publish, presence
• rtm/android.md — RtmClient v2 (Kotlin): init, login, subscribe, publish, event listener
• conversational-ai/agent-toolkit-ios.md — ConversationalAIAPIImpl iOS Swift toolkit
• conversational-ai/agent-toolkit-android.md — ConversationalAIAPIImpl Android Kotlin toolkit
• references/integration-patterns.md — RTC+RTM+ConvoAI init order, UID strategy, channel naming, token matrix, codec selection, cleanup sequence

Content fixes and improvements:

• rtc/react.md: add codec interop note (vp8 recommended; vp9 hardware-limited on older iOS Safari)
• rtc/cross-platform-coordination.md: correct codec table — vp8 default and recommended, vp9 iOS Safari requires iPhone 15 Pro / M3+ hardware, h264 does not scale for multi-user
• rtc/README.md: update codec interop note to match corrected recommendation
• rtm/ios.md, rtm/android.md: add v2 to titles to prevent v1 API misuse
• rtm/README.md: add Platform Scope section (client-side only, all v2, no server/desktop variant)
• rtm/web.md: remove RTM v1 legacy section; wrap constructor in try/catch; token-only login form
• conversational-ai/README.md: add SDK-vs-REST routing table; add RTM channel name = RTC channel name gotcha; scope auth section to direct REST implementors
• conversational-ai/auth-flow.md: scope to REST API implementors; add SDK-skip callout
• testing-guidance/SKILL.md: add RTC React Native, Flutter, RTM Web/iOS/Android mocking patterns; add token renewal section; add table of contents

Housekeeping:

• SKILL.md: bump version to 1.2.0; add Multi-Product Integration entry; expand platform lists for RTC and RTM
• CLAUDE.md: update file structure tree with all new files
• README.md: update products list and file tree to reflect all new files and platforms

What does this PR do?

Checklist

• Freeze-forever test applied to all new inline content — would it still be
  correct if never updated?
• Routing still correct from agora/skills/agora/SKILL.md
• New or changed local links are valid
• No duplicate skill names
• No absolute local paths (/Users/...)
• No hardcoded credentials or API keys
• At least one eval case added or updated in tests/eval-cases.md
  (required for new product/platform additions)
• If adding code generation guidance: testing guidance updated
• scripts/validate-skills.sh passes locally