docs: call out OS argv length limit for large string system_prompt

[tapheret2]

Why

Very large system_prompt strings can fail at process spawn (Argument list too long) before any API call.

Proposal

Document:

• prefer system_prompt={"type":"file","path":...} for large prompts
• platform notes (Linux MAX_ARG_STRLEN)

Acceptance

• Docs note near system_prompt options