feat(cloudflare): allow authored agent bases

[FredKSchott]

Summary

• allow addressable Cloudflare agent modules to export an advanced CloudflareAgent extends Agent base class
• keep Flue's filename-derived generated Durable Object class outermost so bindings, migrations, transport hooks, and recovery ownership remain stable
• document the native SDK boundary and add workerd integration coverage for inherited scheduleEvery() alarm execution

Scope

This intentionally leaves workflow extension, top-level Worker event forwarding, and callback-to-Flue-session helpers for separate product decisions.

Validation

• pnpm run build && pnpm run check:types in packages/runtime
• pnpm run build && pnpm run check:types in packages/cli
• pnpm run test in packages/runtime
• pnpm run test in packages/cli
• pnpm run test:integration:cloudflare in packages/runtime
• pnpm run check:types && pnpm run build in apps/docs
• pnpm run check:types at repository root
• pnpm run check:lint at repository root
• git diff --check origin/main...HEAD

Closes #187

[FredKSchott]

Superseded by a consolidated Cloudflare extensibility PR against main. Closing this stacked PR to simplify review.