feat: add MCP SDK v2 support

[thiskevinwang]

Summary

• migrate client and server helpers to the MCP SDK v2 package split
• serve MCP 2026-07-28 requests with stateless legacy fallback across Hono, Express, and Next.js
• use SDK bearer authentication with Clerk token expiration and scope enforcement
• persist complete OAuth client data, tokens, issuer, refresh token, scope, and discovery state
• add migration guides, framework examples, Node 20 coverage, and protocol tests

Validation

• pnpm fmt:check
• pnpm lint
• pnpm exec tsc --noEmit
• pnpm test (34 tests)
• pnpm build
• pnpm dedupe --check
• pnpm changeset status --since=origin/main