Simplify graceful worker shutdown

[t-kalinowski]

Summary

This makes closing worker stdin the only graceful shutdown request for reset, respawn, and server shutdown. The change removes interpreter-specific shutdown snippets and sideband shutdown delivery so lifecycle behavior is simpler and avoids races between stdin and sideband.

User-Facing Changes

• Reset and shutdown no longer send quit("no"), exit(), or custom shutdown text to worker stdin.
• A user-level prompt such as R readline() or Python input() can no longer consume graceful shutdown text as ordinary input.
• EOF-driven interpreter finalization remains best effort; existing timeout and OS termination escalation remains the fallback.

Internal Changes

• Removes worker_ready.graceful_shutdown.stdin from the worker protocol and built-in R/Python worker-ready messages.
• Removes the server-to-worker sideband shutdown command from graceful shutdown.
• Updates reset/shutdown behavior to close stdin and wait through the existing escalation path.
• Adds public regression coverage for custom Zod workers and Python input() during reset.
• Records the direction as ADR 0001 and updates protocol docs.