Disable Rod leakless launcher on Windows

[Valid-Systems]

Summary

• Disable Rod's leakless launcher helper on Windows while leaving the existing launcher behavior unchanged on other platforms.
• Keep kage's explicit browser cleanup path in place via the existing Pool.Close flow.
• Add a focused regression test for the Windows leakless launcher policy.

Fixes #42.

Test plan

• Applied /wio test . workflow from this wio: selected the Windows launcher policy as the highest-value regression guard, chose the narrowest unit-level oracle for the platform decision, then validated through the real CLI clone surface.
• go test ./browser -run TestLauncherLeaklessDisabledOnWindows
• go test ./browser
• go test -race -shuffle=on -count=1 ./browser
• go build -o D:\tmp\opencode\kage-pr42\kage.exe ./cmd/kage
• D:\tmp\opencode\kage-pr42\kage.exe clone https://example.com --max-pages 1 --force -o D:\tmp\opencode\kage-pr42\out-final-2 succeeded without --control-url; output contained example.com\index.html.
• Defender matching detection count around that clone stayed unchanged: 1 -> 1.
• kage.exe clone --help renders clone help; kage.exe clone ":bad-url" --max-pages 1 --force returns the expected invalid URL error.

Full-suite note

• go test ./... currently has one Windows failure in unmodified pack tests: pack.TestBuildAppDir reports AppRun is not executable at pack/appdir_test.go:64. All other packages/tests passed (103 passed, 1 failed).

[tamnd]

LGTM, thank you for your contribution! Please feel free to close the related issue.