Add Gitea Actions pipeline with basic checks

scripts/check.sh (also `make check`) parses the Swift sources, validates
Info.plist and its placeholders, checks the Makefile VERSION against the
newest CHANGELOG release, and flags whitespace errors and conflict markers.

The lint job runs it on the existing Linux runners in a swift:6.3
container. A full macOS build job is included but skipped until a macOS
runner is registered and the MACOS_RUNNER repo variable is set to true.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Makefile

@@ -15,7 +15,7 @@ CONTENTS      := $(APP)/Contents
 # A secure timestamp is required for notarization but impossible for ad-hoc.
 TS := $(if $(filter -,$(SIGN_ID)),--timestamp=none,--timestamp)
 
-.PHONY: all build universal app app-universal bundle sign install uninstall run clean
+.PHONY: all build universal app app-universal bundle sign install uninstall run check clean
 
 all: app
 
@@ -73,6 +73,10 @@ uninstall:
 run: build
 	"$(BUILD_DIR)/$(APP_NAME)"
 
+## The same basic checks CI runs (syntax, Info.plist, version, whitespace).
+check:
+	scripts/check.sh
+
 clean:
 	swift package clean
 	rm -rf .build "$(DIST)"