Fix agent command resolution on Windows

[RasmusHochreuter]

Summary

• On Windows, subprocess.Popen cannot find executables by bare name (e.g. claude) without the file extension (.exe). This causes ralph run to crash with FileNotFoundError when the agent field in RALPH.md uses a bare command name like claude -p --dangerously-skip-permissions.
• Uses shutil.which() to resolve the executable path before passing it to subprocess.Popen. This works cross-platform — finding claude on Unix and claude.exe on Windows.
• No behavior change on macOS/Linux where bare names already resolve correctly.

Reproduction

On Windows with Claude Code installed:

# RALPH.md frontmatter
agent: claude -p --dangerously-skip-permissions

$ ralph run .
Run crashed: Agent command not found: 'claude -p --dangerously-skip-permissions'
FileNotFoundError: [WinError 2] The system cannot find the file specified

Fix

Added shutil.which() resolution in _run_agent_phase() (engine.py) right after shlex.split() parses the agent command string. If the executable is found on PATH, the resolved absolute path replaces the bare name. If not found, the original value is preserved so the existing FileNotFoundError handling still produces a clear error message.

Test plan

• Verified fix resolves the crash on Windows 11
• Confirm no regression on macOS/Linux (bare names still work)
• Existing tests pass (test_engine.py agent-related tests)

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