fix(windows): use opencode.cmd when spawning OpenCode on win32

[reisen7]

Summary

Fix a Windows-specific issue where Ralph fails to launch OpenCode via Bun.spawn().

Problem

On Windows, opencode may work from an interactive shell, but Ralph can still fail with:

❌ Error in iteration 5: 2045 |         allowAllPermissions: allowAllPermissions,
2046 |       });
2047 |
2048 |       // Run agent using spawn for better argument handling
2049 |       // stdin is inherited so users can respond to permission prompts if needed
2050 |       currentProc = Bun.spawn([agentConfig.command, ...cmdArgs], {
                               ^
ENOENT: no such file or directory, uv_spawn 'opencode'
    path: "opencode",
 syscall: "uv_spawn",
   errno: -4058,
    code: "ENOENT"

      at runRalphLoop (xxxx\node_modules\@th0rgal\ralph-wiggum\ralph.ts:2050:25)

Continuing to next iteration...

Root cause

In some Windows environments, the executable entry point is exposed through the opencode.cmd shim.
Shells can resolve this automatically, but direct spawning with Bun.spawn() may not.

Fix

When running on win32, if the configured command is opencode, use opencode.cmd instead.

Validation

• Verified opencode --version works in the terminal
• Verified Ralph can launch OpenCode successfully on Windows
• Confirmed the previous uv_spawn 'opencode' failure no longer occurs