feat(passkey-crypto): add registerPasskey()

[derranW26]

https://linear.app/bitgo/issue/WCN-188/implement-registerpasskey

Summary

• Adds registerPasskey() to @bitgo/sdk-core — registers a passkey at the BitGo account level
• Types (WebAuthnOtpDevice, WebAuthnProvider, PasskeyAuthResult, PasskeyGetOptions) are re-exported from @bitgo/passkey-crypto (no local stubs)
• Exports the new passkey module from sdk-core/src/bitgo/index.ts
• Updates Dockerfile to include @bitgo/passkey-crypto

Flow

1. GET /api/v2/user/otp/webauthn/register — fetch server challenge (contains baseSalt)
2. provider.create() — browser returns attestation
3. If PRF output present → include scopes: ['prf'] in PUT payload; otherwise omit
4. PUT /api/v2/user/otp — register the passkey
5. Returns WebAuthnOtpDevice & { prfSupported: boolean }

Acceptance criteria

• baseSalt sourced from server challenge — never generated client-side
• Calls GET before provider.create()
• PRF output → scopes included; no PRF → scopes omitted
• Returns prfSupported: boolean
• Uses PUT — not POST
• No any types
• Unit tests cover both PRF branches + call ordering

Test plan

• Unit tests cover PRF-present branch (scopes included, prfSupported: true)
• Unit tests cover PRF-absent branch (scopes omitted, prfSupported: false)
• Unit test asserts GET challenge fires before provider.create()

TICKET: WCN-188

[linear-code]

WCN-188 Implement `registerPasskey()`

[derranW26]

@claude review this pr