chore: revert reject unsupported task system_prompt

[mikasenghaas]

Summary

Cleanly reverts #1735 ("fix: reject unsupported task system_prompt"), restoring the prior Harness.resolve_prompt behavior:

• A harness without APPENDS_SYSTEM_PROMPT (e.g. codex / kimi_code / mini_swe_agent) once again folds task.system_prompt into the user prompt ("{system}\n\n{prompt}", with a warning) instead of raising a ValueError.
• APPENDS_SYSTEM_PROMPT harnesses (default / bash / rlm) are unaffected — the system prompt is still emitted separately.
• Restores the original docstrings on APPENDS_SYSTEM_PROMPT, resolve_prompt, and Task.system_prompt.

Produced with git revert cc4afd52 (the #1735 squash-merge); a clean inverse with no manual edits. The later setup method and boundary-error changes (#1749 / #1751) are untouched.

Breaking

• Reverts the breaking change from fix: reject unsupported task system_prompt #1735: tasks that set system_prompt on a non-APPENDS_SYSTEM_PROMPT harness no longer error — they fold the system prompt into the user message (with a warning) as before.

Note

Revert rejection of tasks with unsupported system prompts in Harness.resolve_prompt

• When a harness does not append system prompts (APPENDS_SYSTEM_PROMPT=False) and the prompt is a plain string, resolve_prompt now logs a warning and prepends the system prompt to the user prompt instead of raising an error.
• A ValueError is still raised when the prompt is Messages or None and a system prompt is provided, as folding is not possible in those cases.
• Updates Task.system_prompt field docs in task.py to reflect the new prepend-with-warning behavior.

^{Macroscope summarized 2bccead.}