fix: invoke zeroshot via node on Windows

[haosenwang1018]

Summary

• add a small buildZeroshotSpawnSpec() helper for self-invocation
• on Windows, spawn process.execPath with cli/index.js instead of trying to execute bare zeroshot
• reuse that helper in both agent-task-executor and claude-task-runner
• add regression coverage for the Windows vs non-Windows spawn spec

Why

Issue #457 reports that spawn('zeroshot', ...) fails on Windows because npm global commands are exposed as .cmd wrappers, which Node cannot execute the same way as Unix binaries.

This PR fixes the smallest high-impact part first: zeroshot spawning itself. On Windows it now invokes Node directly with the CLI script path, avoiding the .cmd wrapper and the resulting ENOENT failures.

Testing

• ./node_modules/.bin/mocha tests/providers/detection.test.js

Part of #457