Ship skit Linux release bundles

[leogdion]

The library and skit executable already compile on Linux (Subprocess is conditional on .linux, OutputCache has an XDG/~/.cache/syntaxkit branch, String+DylibFilename returns .so). Once the OutputCacheTests ProcessInfo-subclass fix lands (PR #156 follow-up), CI should go green — but we don't yet produce a runnable release bundle on Linux.

Scope

• Add a Linux branch to Scripts/build-skit.sh (or a dedicated build-skit-linux.sh):
  • Glob .build/<linux-triple>/{debug,release} instead of .build/*-apple-macosx*/{debug,release}.
  • Copy libSyntaxKit.so instead of .dylib.
  • Replace install_name_tool -id "@rpath/libSyntaxKit.dylib" with patchelf --set-rpath '\$ORIGIN' on the binary (or --force-rpath / RUNPATH equivalents). Verify ldd resolves libSyntaxKit.so to the bundled lib/.
  • Skip strip -x (Mach-O specific) — use strip --strip-unneeded for the release equivalent.
• Update RunCommandError.toolchainMismatchMessage and the help text in Skit+Run.swift to reference the right script path per platform (or a single platform-agnostic path that the script dispatches internally).
• Add a CI step that runs the produced bundle end-to-end on Ubuntu (`./.build/skit-release/skit run Examples/Completed/card_game/dsl.swift`) — current Ubuntu jobs only `swift build`/`swift test`.
• Confirm `_SwiftSyntaxCShims-include` copy still works (path is the same on Linux).
• Document Linux bundle layout in `Docs/skit.md`.

Out of scope

• Windows support (tracked separately).
• A Linux Homebrew-equivalent distribution channel.

Risks

• `swift-subprocess`'s Linux backend exercise — needs a sanity smoke test beyond what current CI does.
• `patchelf` is the standard but not always pre-installed on minimal images; document or auto-install.