feat(internet-identity): update for @icp-sdk/auth v6 API

[sea-snake]

Summary

Updates the internet-identity skill to match the new @icp-sdk/auth v6 API.

• AuthClient.create() becomes new AuthClient(options); identityProvider is now passed to the constructor.
• Callback-based login({ onSuccess, onError }) becomes promise-based await signIn({ maxTimeToLive }) returning the Identity directly.
• isAuthenticated() is now sync; getIdentity() is now async.
• Pitfall Migrate @dfinity/* packages to @icp-sdk/* #3 reworded around the new promise-based flow; pitfall Improve repo structure, CI pipeline, and contributor experience #5 updated to mention the new failure modes.
• Code example rewritten end-to-end against the new API. Adds a comment about openIdProvider for one-click sign-in (Google / Apple / Microsoft).
• Bumps prerequisite to @icp-sdk/auth >= 6.0.0.

npm run validate passes (warnings only, all pre-existing missing-evaluations warnings unrelated to this change).

Companion PR: dfinity/developer-docs#189 updates the consumer page that this skill backs.

[github-actions]

Thank you for contributing! Unfortunately this repository does not accept external contributions yet.

We are working on enabling this by aligning our internal processes and our CI setup to handle external contributions. However this will take some time to set up so in the meantime we unfortunately have to close this Pull Request.

We hope you understand and will come back once we accept external PRs.

— The DFINITY Foundation