Implement cross-platform process cleanup handlers for LSP servers.
Added registerProcessCleanup() method to LSPServerManager that:
- Kills all spawned LSP server processes on process.exit
- Handles SIGINT (Ctrl+C) - all platforms
- Handles SIGTERM (kill signal) - Unix/macOS/Linux
- Handles SIGBREAK (Ctrl+Break) - Windows specific
This prevents LSP servers from becoming orphan processes when opencode terminates unexpectedly.
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