feat(cloudflare): allow authored worker extensions

[FredKSchott]

Summary

• discover an optional source-root cloudflare.ts module and re-export its named exports from the generated Cloudflare Worker entry
• compose optional non-HTTP Worker handlers while keeping app.ts as the sole HTTP fetch() customization surface
• replace Sandbox suffix auto-wiring with explicit cloudflare.ts exports and migrate first-party guidance/examples
• reject generated export and binding collisions, and avoid partially updating generated deployment inputs when Wrangler validation fails

Stack

• targets feat/cloudflare-agent-extension / feat(cloudflare): allow authored agent bases #192
• retarget to main after feat(cloudflare): allow authored agent bases #192 merges

Validation

• pnpm run build && pnpm run check:types && pnpm run test in packages/runtime
• 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/feat/cloudflare-agent-extension...HEAD

[FredKSchott]

Superseded by a consolidated Cloudflare extensibility PR against main.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