fix: defer VS Code auto theme to platform - #841

Good, focused change. The reasoning in the PR body matches the diff: when window.autoDetectColorScheme is true, VS Code is following the OS theme, so inferring from workbench.preferredDarkColorTheme/preferredLightColorTheme was wrong — it would always land on whichever theme was configured, ignoring actual OS state. Returning null and letting the caller fall through to platform detection is the right fix, and it's a one-function change in cli/src/utils/theme-system.ts that's easy to review.

The added test in cli/src/__tests__/utils/theme-system.test.ts covers both the explicit-theme path and the auto-detect-defers-to-platform path, which is exactly what the repo would want for this kind of regex-based parser.

One thing to double check before porting: previously, if auto-detect was enabled but the caller's platform detection failed (e.g., non-macOS/unsupported OS), the code would fall back to inferring from preferredDarkColorTheme/preferredLightColorTheme as a best-effort guess. Now it always returns null in that case, so the caller needs a sensible final fallback (e.g., default theme) for those situations, or you risk a regression on platforms without OS-level detection support. Worth confirming the caller (getSystemTheme or wherever this is consumed) handles that gracefully.

Also, minor: exporting extractVSCodeTheme for testing is fine but check whether the codebase has a convention (e.g., barrel test-only exports) for exposing internals like this.

Overall: correct root-cause fix, small diff, includes tests. Worth porting with a quick check of the fallback behavior.