fix: make ClaudeCliBackend work on Windows (.cmd shim resolution + argv length)

[codeL1985]

Problem

On Windows, claude installed via npm is a .cmd shim. subprocess.run(["claude", ...]) raises FileNotFoundError (WinError 2) because CreateProcess does not resolve .cmd files by bare name. The bare except Exception: return "" in ClaudeCliBackend._call swallows this, so every backend call silently returns an empty string — real-backend cycles report held-out 0.000 -> 0.000 => reject with zero edits, which is indistinguishable from "nothing to optimize" (same user-facing symptom as #68, different cause).

Fix

• Resolve claude_path via shutil.which() in __init__ — it honors PATHEXT and returns the full claude.CMD path.
• Pass the prompt via stdin instead of argv, in both _call and attempt_with_tools. The .cmd shim routes through cmd.exe, whose command line caps at ~8K chars; reflect/judge prompts routinely exceed that. claude -p with no positional prompt reads stdin, so behavior is unchanged on POSIX.

Verification (Windows 11, Python 3.14, claude CLI 2.1.170)

• Before: reflect() on a synthetic max_chars failure returned [] (raw _call output was "").
• After: same call returns a concrete bounded edit through the real CLI; a 12KB stdin prompt round-trips cleanly (rc 0).

🤖 Generated with Claude Code

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