document native mobile passkey setup

[mdfarhankc]

Summary

Adds a Native mobile apps (iOS / Android) section to the skill passkeys reference. The library already handles native-app origin formats via py-webauthn; this is purely documentation for adopters integrating Flutter / React Native / native iOS-Android clients.

Covers:

• Why public-suffix domains (*.vercel.app, *.netlify.app, *.github.io, *.herokuapp.com) are a poor fit for native-app passkeys and why a custom domain is effectively required before App Store / Play Store release.
• apple-app-site-association and assetlinks.json setup with working JSON samples.
• Origin format per platform — iOS piggy-backs on the web origin, Android requires an android:apk-key-hash:<base64url(SHA-256)> entry in PASSKEY_ORIGINS.
• Flutter client setup (iOS associated-domains entitlement, Android Credential Manager).
• Common pitfalls: Play App Signing fingerprint vs upload keystore, AASA content-type, RP ID case mismatch, preview deployment URL drift.

No code changes.

Test plan

• uv run ruff format --check . clean
• uv run ruff check . clean
• uv run pytest tests/ — 188 passed, 1 skipped (no code changes, tests unaffected)
• CI green on this PR

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: bb9570ed-9697-4a68-9759-7fe253cbae4b

📥 Commits

Reviewing files that changed from the base of the PR and between 3aa9270 and db6ca92.

📒 Files selected for processing (1)

• fastapi_fullauth/.agents/skills/fastapi-fullauth/references/passkeys.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/passkey-native-mobile

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