Devbox entrypoint behavior (cn serve)

Context: runloop resumes a devbox by re-running the same entrypoint script, which invokes cn serve --id <agentId> .... Because the entrypoint always replays, the CLI must avoid duplicating state on restart.

• Session reuse: serve now calls loadOrCreateSessionById when --id is provided so the same session file is reused instead of generating a new UUID. This keeps chat history intact across suspend/resume.
• Skip replaying the initial prompt: shouldQueueInitialPrompt checks existing history and only queues the initial prompt when there are no non-system messages. This prevents the first prompt from being resent when a suspended devbox restarts.
• Environment persistence: The devbox entrypoint (control-plane) writes all env vars to ~/.continue/devbox-env and sources it before cn serve, so keys survive suspend/resume. The CLI assumes env is already present.

Operational notes:

• Changing the entrypoint is expensive; prefer adapting CLI/session behavior as above.
• When testing suspend/resume, confirm a single session file under ~/.continue/sessions for the agent id and that follow-up messages append normally without replaying the first prompt.