Update lint.yml

[oliexe]

Note

Low Risk
Low risk CI-only change; it broadens lint coverage and adds directory-existence guards, with the main risk being unexpected CI failures if formatting or shellcheck issues exist in the newly included paths.

Overview
Updates the GitHub Actions Lint workflow to run dotnet format whitespace against the UPM package (Packages/gg.stash.unity) and optionally Assets when those directories exist.

Expands ShellCheck scanning to include Packages/**/*.sh in addition to existing Assets and .github scripts.

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