TUI: treat interrupted streams as clean aborts

[paulbettner]

• Handle ESC interrupts as user aborts instead of unknown errors.
• Detect abort signal and OpenAI “reasoning … required following item” error; store MessageAbortedError (“Request was aborted”).
• Clean up open reasoning parts; preserve tool input; mark tools “Tool execution aborted”.
• Add unit test and interactive scripts; typecheck passes.

Files

• packages/opencode/src/session/prompt.ts
• packages/opencode/test/interrupt-handling.test.ts
• test-interrupt-*.sh