Add shell script diagnostics support (shellcheck)

[aakashrajput]

What

Same file/mechanism as the Ruby diagnostics issue (src/tools/diagnostics.ts, commandFor()) — .sh/.bash files currently get no diagnostics at all.

Where to start

if ([".sh", ".bash"].includes(ext)) {
  if (onPath("shellcheck")) return ["shellcheck", "-f", "gcc", absPath];
  return null;
}

Acceptance criteria

Same shape as the Ruby issue: silent skip when shellcheck isn't installed, never blocks, add a couple of unit tests.

Why it's a good first issue

Independent of the Ruby diagnostics issue — pick whichever language you'd rather work with; both are separate, non-conflicting PRs.

[Ayush7614]

Hi @aakashrajput — I created a PR for this: #44

It adds shellcheck diagnostics for .sh/.bash/.zsh, and also extends the same PATH-gated pattern to Swift (swiftlint), PHP (phpstan/pint/php -l), and Kotlin (ktlint).