Windows: SDK should fall back to --system-prompt-file when cmdline overflows (follow-up to #501)

## Summary

The Python SDK still hits Windows' 32,767-char `CreateProcess` limit for `system_prompt`, even though the CLI now exposes `--system-prompt-file ` as a workaround. The SDK should fall back to a temp file (same pattern it already uses for `--agents`) when the resolved command line would exceed the platform limit.

Filing as a follow-up because:
- #501 (closed 2026-03-26) covered this for both `--agents` (SDK already temp-files, CLI doesn't read `@`) and `--system-prompt` (no fallback at all). Closed without resolution and no "fixed in vX.Y" reference.
- #267 was closed as duplicate of #501.
- Since then, the CLI has gained `--system-prompt-file` (visible in `claude --help` under `--bare`'s flag description), so the half that needs to land is purely SDK-side.

## Reproduction

Confirmed today against:
- `claude_agent_sdk` 0.1.x (latest pip install, traceback from `subprocess_cli.py:485`)
- `claude.exe` 2.1.119
- Python 3.14.3 on Windows 11 (Windows workstation)

The triggering call enriches `ClaudeAgentOptions.system_prompt` with RAG-injected wiki chunks and memory hits, totaling ~48,755 chars. Result:

```
File "C:\Users\Jaime\AppData\Local\Programs\Python\Python314\Lib\subprocess.py", line 1552, in _execute_child
hp, ht, pid, tid = _winapi.CreateProcess(executable, args, ...)
FileNotFoundError: [WinError 206] The filename or extension is too long

# Then the SDK relabels it:
claude_agent_sdk._internal.transport.subprocess_cli.py:485 in connect
raise error from e # error = CLINotFoundError(f"Claude Code not found at: {self._cli_path}")
```

Two problems compounded here:

1. **The actual cause is unrelated to the binary.** The CLI binary exists, runs fine standalone (`claude.exe --version` returns `2.1.119`), and the `_find_cli` path resolution worked — the spawn just fails because `--system-prompt <48K>` blows the cmdline. Python's `subprocess` raises `FileNotFoundError` for `WinError 206`, which the SDK's `except FileNotFoundError` block at `subprocess_cli.py:484` mistakes for a missing binary.

2. **No SDK fallback exists.** The SDK already has the `--agents` temp-file pattern at `subprocess_cli.py:336-362`. It does not extend it to `--system-prompt`, so callers have no out short of pre-truncating on their side.

## Suggested fix

Two targeted changes:

### 1. Mirror the `--agents` temp-file fallback for `--system-prompt`

When the resolved `cmd` string exceeds `_CMD_LENGTH_LIMIT`, write `--system-prompt`'s value to a `tempfile.NamedTemporaryFile`, replace it with `--system-prompt-file `, and clean up after the subprocess exits. Same shape as the existing `--agents` branch. The CLI flag is now real, so the value will land correctly without `@filepath` games.

```python
# subprocess_cli.py — pseudocode, parallel to the existing --agents block
cmd_str = " ".join(cmd)
if len(cmd_str) > _CMD_LENGTH_LIMIT:
if "--system-prompt" in cmd:
idx = cmd.index("--system-prompt")
text = cmd[idx + 1]
tf = tempfile.NamedTemporaryFile(mode="w", suffix=".txt",
delete=False, encoding="utf-8")
tf.write(text); tf.close()
cmd[idx] = "--system-prompt-file"
cmd[idx + 1] = tf.name
self._tempfiles_to_cleanup.append(tf.name)
```

Same logic for `--append-system-prompt` → `--append-system-prompt-file`.

### 2. Distinguish "binary missing" from "spawn failed for another reason"

The current `except FileNotFoundError` block treats any `FileNotFoundError` from `_winapi.CreateProcess` as "CLI not found at path". On Windows, that is a lie for at least three error codes that Python surfaces as `FileNotFoundError`: `ERROR_FILE_NOT_FOUND` (2), `ERROR_PATH_NOT_FOUND` (3), and `ERROR_FILENAME_EXCED_RANGE` (206). The error message we got steered me into a 90-minute investigation of a missing-binary that wasn't missing.

Two options:

- **Cheap**: before raising `CLINotFoundError`, double-check `os.path.isfile(self._cli_path)`. If True, raise `CLIConnectionError` with a more informative message (include cmdline length + platform limit).
- **Better**: catch `OSError` instead of just `FileNotFoundError`, inspect `e.winerror` on Windows, and branch on `206` → "Command line exceeded Windows limit ({len} chars vs 32,767)" before falling through to the file-not-found branch.

The "cheap" option alone would have saved my afternoon.

## Why this matters in practice

The SDK is the recommended Python integration path. Anyone building a NetOps/observability/RAG-grounded bot on Windows that injects retrieved context into `system_prompt` (which is the natural place for it) will hit this on the first non-trivial query. It silently looks like a binary-installation problem.

The fix is mechanical (pattern already in the codebase for `--agents`) and the user-facing payoff is large.

## Environment

- OS: Windows 11 Home 26200
- Python: 3.14.3 (per-user install)
- `claude_agent_sdk`: latest pypi
- `claude.exe`: 2.1.119
- Run mode: subprocess CLI transport (default)
- Caller: NemoBot service (NSSM-managed Python process running as local user)

## Related

- #501 — covers both halves; closed without explicit resolution
- #267 — closed as duplicate of #501
- #45935 in `anthropics/claude-code` — `--append-system-prompt-file` not cumulative (CLI side; orthogonal but confirms the file flag is real)