fix(runner): avoid stdin backpressure before output readers start

[Haloukaidi]

Prompt delivery currently writes synchronously to the child's stdin before stdout/stderr readers and watchdog checks start. A child that emits enough startup output before reading a long prompt can fill both pipe buffers and leave both processes waiting.

Provide the UTF-8 prompt through a seeked temporary file inherited as stdin. The parent does not write into a live child's pipe; newline/EOF behavior and prompt privacy are retained. The temporary parent handle is closed when spawning finishes, including spawn failure. Existing file-based prompt paths use closed input through DEVNULL.

Validation on Windows / Python 3.13:

• 38 tests passed across new stdin backpressure cases, Claude prompt delivery, and stream callbacks.
• Real subprocess cases cover large ASCII and Unicode prompts with large startup stdout/stderr, stop/hard-idle while the child never reads input, and temporary-input cleanup on spawn failure.
• Ruff and git diff --check passed.

Draft pending cross-platform review and CI. This change is independent of Windows descendant ownership and output-reader cleanup.