build(Sky): Fix Bundled variant build by using dynamic imports and Vite stub plugin

Convert static imports of Bundled/*/Layout.astro components to dynamic `await import()` in Astro frontmatter. The previous static imports forced Rollup to walk every variant's VS Code module graph, including Browser's `workbench.web.main.internal.js` which exists only in the gulp-only `out/` tree and carries unmangled symbol references that cannot resolve against Output's release `out-build/` tree (mangled, telemetry-stripped).

Also add the `BundledEntryStubInactive` Vite plugin in astro.config.ts that replaces inactive variant Entry.ts modules with an empty stub at `load()` time. This short-circuits Rollup's graph walk for variants not in the active `Pack` env var while leaving the active variant's Entry bundled normally through `BundledInputs`.

The `SkyChannels.ts` auto-generated file is also included, documenting all 59 `sky://` IPC channels Mountain can emit and whether Sky has listeners for each.

Author: NikolaRHristov