[codex] Fix Windows screenshot DPI and display capture

[fffonion]

Summary

Fixes Windows screenshot capture behavior in the curated screenshot skill.

This commit is drafted and created by Codex.

Changes

• Enables per-monitor DPI awareness before reading Windows display and window rectangles.
• Uses DWM extended frame bounds for window captures, with GetWindowRect as a fallback.
• Adds milliseconds to generated filenames to reduce same-second collisions.
• Updates the skill documentation for the new Windows behavior.

Validation

• Ran work/test_screenshot_windows.ps1 against the curated skill helper on a mixed-DPI three-display Windows setup.
• Verified default capture size matched the virtual desktop.
• Verified a Codex window capture matched DWM visible frame bounds.