fix(process): prevent orphaned opencode subprocesses on shutdown

[thdxr]

Summary

• make TUI thread shutdown idempotent and timeout-bounded so worker teardown cannot block process exit
• always terminate the TUI worker and unregister process listeners during cleanup
• harden MCP local client teardown (including BUN_BE_BUN opencode subprocesses) with timeout-bounded close and process-tree termination fallback

Root Cause

• some shutdown paths relied on RPC/client close calls that could hang indefinitely when a worker/subprocess became unhealthy
• when MCP local startup or reconnect failed, child processes could outlive the parent connection lifecycle

Changes

• packages/opencode/src/cli/cmd/tui/thread.ts
  • centralize shutdown in an idempotent stop() function
  • enforce bounded shutdown with timeout and guaranteed worker.terminate()
  • simplify flow using helper functions for worker path and prompt text
• packages/opencode/src/mcp/index.ts
  • add closeClient() helper that:
    • attempts graceful close with timeout
    • signals descendant processes
    • escalates to stronger termination if needed
  • use closeClient() consistently on dispose, reconnect/replace, disconnect, and failed startup/tool-list paths

Testing

• bun run typecheck
• bun test test/cli/tui/transcript.test.ts
• bun test test/mcp/headers.test.ts test/mcp/oauth-browser.test.ts