fix: clear error when system_prompt hits OS argv length limit

[tapheret2]

Summary

When system_prompt is a large string, Linux fails at execve with "Argument list too long" (per-argv MAX_ARG_STRLEN = 32 * PAGE_SIZE) before any API request.

This PR checks the encoded size against the platform limit (via os.sysconf("SC_PAGE_SIZE") when available) and raises CLIConnectionError that:

• names system_prompt
• reports measured size and the limit
• points to the working file form: system_prompt={"type": "file", "path": "..."}

Windows / platforms without sysconf skip the preflight (the OS limit does not apply the same way).

Closes #1096