0.2.0: native Chrome compatibility, E2E harness, docs & release

[kashishhora]

Summary

Final PR of the WebMCP 0.2.0 alignment (follows #23 and #24). Adds native-browser verification, the compatibility fixes it surfaced, the docs/CHANGELOG, the version bump, and migrates the in-repo examples/skills.

• Native E2E harness (examples/native-harness): a Vite + React app that registers tools, shows native-vs-polyfill detection and a live toolchange counter, and a one-click self-test driving navigator.modelContextTesting. Verified against native document.modelContext in Chrome 151 (detection native, tools register, executes PASS, clean console).
• Native compatibility fixes found via that harness:
  • registerTool tolerates native Chrome (≤151), which returns undefined and throws synchronously rather than returning a Promise. The hook now handles both the native (void + sync throw) and spec/polyfill (rejecting Promise) shapes instead of crashing on .catch.
  • An already-aborted AbortSignal now resolves as a no-op (registration skipped), matching native Chrome 151. (Spec PR #202 specifies rejection; native hadn't shipped that — revisit if it changes.)
• Docs + release: rewritten docs/api.md, README "Breaking changes in 0.2.0", updated AGENTS.md, new CHANGELOG.md, and version bump to 0.2.0.
• Examples + skills migrated to the 0.2.0 API (playground, Next.js example, webmcp-add-tool / webmcp-setup skills).

Test plan

• pnpm typecheck / pnpm lint — clean
• pnpm test — 174 passing
• pnpm build, pnpm --filter webmcp-react-playground build, native-harness build
• Manual native verification in Chrome 151 Canary (#enable-webmcp-testing): self-test PASS against native document.modelContext

Merging this completes 0.2.0 and bumps the version; publish to npm after it lands.