Read an empty example Bot PORT as unset, so NaN never reaches Bun.serve

[Bartok9]

What this changes

The LangGraph and Mastra example Bots used Number.parseInt(process.env.PORT ?? default). An empty PORT= is "", not unset, so they bound an ephemeral port while docs still named 4300/4400. They now share listenPort with agent-bot (#395).

Where it runs

• New state that outlives a request? None.
• What happens on the second replica? Each process still reads its own PORT at boot. Empty → documented default; a prefix typo still refuses to start.
• Anything serialised? No.
• Anything fanned out to a browser? No.
• New listener, port, or schedule? No. Same listen, same documented ports.

Boundary and audit

• Gateway path unchanged.
• No new refusals in the server audit trail (process exits at boot, same as agent-bot).
• Nothing new trusted from the client.

Changelog

• Unreleased: example LangGraph/Mastra Bots no longer bind an ephemeral port on empty PORT=.

Proof

shared/listen-port.test.ts already covers empty/prefix/range. Same helper, same defaults 4300/4400.