[CI] Add shellcheck linting workflow

[passwordless-OTP]

Add GitHub Actions workflow to run shellcheck on all plugins.

Requirements

• shellcheck all .plugin.zsh files
• Fail on warnings
• Run on PRs
• Run on main branch pushes

Workflow

name: Lint
on: [push, pull_request]
jobs:
  shellcheck:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - name: Run shellcheck
        run: |
          find . -name '*.plugin.zsh' -exec shellcheck {} \;

/label ci,enhancement
/milestone v0.2