Run the shared quality checks on pull requests

Call the reusable quality-check workflow of TirSystem/github-action for the
shell files: shellcheck and shfmt -d. Add switch_case_indent to the
[*.sh] section of .editorconfig so that shfmt, which reads it, agrees with
the style the project already uses (shfmt -i 2 -ci in tests/run-tests.sh).

.editorconfig

@@ -7,3 +7,6 @@ end_of_line = lf
 insert_final_newline = true
 charset = utf-8
 trim_trailing_whitespace = true
+
+[*.sh]
+switch_case_indent = true

.gitea/workflows/quality.yml

@@ -0,0 +1,18 @@
+name: Quality
+
+# Calls the shared quality-check workflow of TirSystem/github-action
+# (contract: ADR-0005 there). It runs shellcheck and shfmt on the tracked
+# shell files and changes no file. Configuration comes from .shellcheckrc
+# and .editorconfig in this repository.
+on:
+  pull_request:
+  push:
+    branches: [ main ]
+
+jobs:
+  quality:
+    permissions:
+      contents: read
+    uses: TirSystem/github-action/.gitea/workflows/quality-checks.yml@main
+    with:
+      languages: shell