Allow passing system prompts as files.

[valayDave]

When we craft very long files as system prompts (ones that can easily fit in context), claude's python SDK breaks down with an error like

Traceback (most recent call last):
  File "/opt/outerbounds/fb/lib/python3.12/asyncio/runners.py", line 194, in run
    return runner.run(main)
           ^^^^^^^^^^^^^^^^
  File "/opt/outerbounds/fb/lib/python3.12/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/outerbounds/fb/lib/python3.12/asyncio/base_events.py", line 664, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/metaflow/claude_utils.py", line 232, in _query_async
    async with ClaudeSDKClient(options=self.options) as client:
  File "/opt/outerbounds/fb/lib/python/site-packages/claude_agent_sdk/client.py", line 329, in __aenter__
    await self.connect()
  File "/opt/outerbounds/fb/lib/python/site-packages/claude_agent_sdk/client.py", line 132, in connect
    await self._transport.connect()
  File "/opt/outerbounds/fb/lib/python/site-packages/claude_agent_sdk/_internal/transport/subprocess_cli.py", line 276, in connect
    raise error from e
claude_agent_sdk._errors.CLIConnectionError: Failed to start Claude Code: [Errno 7] Argument list too long: '/usr/local/share/npm-global/bin/claude'

This is happening because the entire system prompt is passed to the claude's NPM package via CLI. If we can pass system prompts via files, this would easily circumvent the issue.

People also recommend telling claude to read a specific file in the system prompt. But this is not what I want. I want that I can add as much as I can to the system prompt.