feat(cloudflare): add authored deployment and agent extensions

[FredKSchott]

Summary

• add colocated addressable-agent extensions through cloudflare = extend({ base, wrap }), with native Agents SDK subclasses beneath Flue-owned routing and prototype-preserving wrappers such as Sentry around the final generated Durable Object class
• discover an optional source-root cloudflare.ts module whose named exports become Worker exports and whose optional default export contributes non-HTTP handlers while app.ts remains the HTTP composition surface
• replace implicit Cloudflare Sandbox suffix wiring with explicit deployment-module exports
• preserve stable Wrangler class names and Agents SDK runtime identities, reject generated export and binding collisions, and validate generated deployment inputs before writing

Agent-local extension example

import * as Sentry from '@sentry/cloudflare';
import { createAgent } from '@flue/runtime';
import { extend } from '@flue/runtime/cloudflare';

export default createAgent(() => ({
  model: 'anthropic/claude-sonnet-4-6',
}));

export const cloudflare = extend({
  base: (Base) =>
    class extends Base {
      async onStart() {
        await this.scheduleEvery(60, 'heartbeat');
      }

      async heartbeat() {
        this.setState({ ...this.state, lastHeartbeatAt: Date.now() });
      }
    },
  wrap: (Final) =>
    Sentry.instrumentDurableObjectWithSentry(
      (env: Env) => ({ dsn: env.SENTRY_DSN }),
      Final,
    ),
});

Review notes

• this consolidates and supersedes feat(cloudflare): allow authored agent bases #192 and feat(cloudflare): allow authored worker extensions #193
• agent-local cloudflare descriptors are separate from the source-root cloudflare.ts deployment module
• base may return the received class or a subclass; wrap may return the received class or a prototype-preserving constructor proxy
• workflow extension and FlueRegistry wrapping remain intentionally out of scope

Validation

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

Closes #187