fix(web): terminate Windows PTYs without unsupported signals

[outlier27-cell]

Problem

On Windows, node-pty terminates through ConPTY and rejects POSIX signal arguments. The Web interactive terminal manager passed SIGKILL on force cleanup, and scheduled a POSIX force-kill fallback after normal cleanup.

Value

Windows Web terminals can close, be retained, and be disposed without throwing from node-pty. This prevents terminal lifecycle cleanup from failing on Windows while preserving the existing POSIX escalation behavior.

Approach

Use signal-free pty.kill() on Windows and keep the existing SIGKILL escalation only on non-Windows platforms. Add Windows-specific fake-PTY coverage for close, retain, and dispose, plus a native Windows PTY exit test.

Validation

• bun run check passed.
• node --experimental-strip-types --test tests/web/interactive-terminal.test.ts: 7/7 passed on Windows.
• git diff --check passed.
• Full bun run test was not rerun for this isolated two-file change; the repository baseline has unrelated post-edit lifecycle failures documented in PR fix(setup): recover locks while writers wait #604.

Impact

• User-visible behavior: Windows Web terminal cleanup now completes reliably.
• Model-visible context/tools: none.
• Runtime/lifecycle: Windows PTYs use the native signal-free termination API; POSIX escalation is unchanged.
• Persisted config/data: none.
• Compatibility or risk: change is limited to Windows PTY cleanup and its tests.

[tt-a1i]

复审提交 819d790083cccee8ff662e1a318598c6b32fb480。

未发现阻断问题。Windows 路径改为 ConPTY 支持的无信号 pty.kill()，并只在 POSIX 保留延迟 SIGKILL 升级；close、retain、dispose 的 fake PTY 用例和原生 Windows PTY 退出用例覆盖了各条清理路径。当前 Node、Web E2E 和原生 Windows 检查均通过。

验证边界：没有重新执行长期运行的真实交互终端压力测试；结论覆盖本 PR 的两文件生命周期变化。