fix(serve): add signal handlers for graceful shutdown on Windows

[JosXa]

Summary

Fixes #9859 - Adds signal handlers for graceful shutdown in opencode serve to prevent ghost port bindings on Windows when the process is terminated by external tools like openralph/opencode-ralph.

Changes

• packages/opencode/src/cli/cmd/serve.ts: Added signal handlers for SIGINT, SIGTERM, and SIGBREAK (Windows-only) that call server.stop() before exiting
• packages/opencode/src/server/server.ts: Added reusePort: true to Bun.serve() for defense-in-depth, allowing immediate port reuse even after abnormal termination

Root Cause

The opencode serve command lacked signal handlers to catch termination signals. When plugins like openralph send SIGTERM to terminate the server, the process would die immediately without calling server.stop(), causing:

1. TCP listener not closed properly
2. Port binding persists as a "ghost" on Windows
3. Server restart fails until port times out (2-5 minutes) or system reboot

Solution

1. Signal handlers: Register handlers for SIGINT, SIGTERM, and SIGBREAK (Windows Ctrl+Break) that gracefully shut down the server
2. reusePort: Enable SO_REUSEPORT on the server socket, allowing immediate port reuse as a fallback for edge cases

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found

[rekram1-node]

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