Fix path.resolve() discarding staging prefix for leading-slash dest

[cewert]

PR #245 replaced string interpolation with path.resolve() in copyToStaging() and zipFolder(). When dest starts with /, path.resolve() silently discards the base staging path, causing EACCES: permission denied, mkdir '/source'.

Changes

• Replace path.resolve() in copyToStaging() with defensive logic that checks if dest is already an absolute path under stagingPath (use as-is) or otherwise strips leading slashes and uses path.join()
• Apply the same pattern in zipFolder() for zip entry path construction
• Add 5 regression tests covering leading-slash dest values in both functions (stubbed and integration)
• Maintain 100% code coverage

[TwitchBronBron]

Hey, thanks for this. I think the bug in the VS Code extension was caused by something slightly different, so I’m not sure this is the implementation I want to add. The absolute change made in #245 supports writing things outside of outDir and rootDir, which was necessary for when vscode users want to set outDir somewhere above the workspace folder. I think this PR would introduce some additional restrictions that we don't actually need.

I just opened #249 and rokucommunity/roku-debug#329 to resolve the vscode issue.