Add justfile task runner

Recipes: check (lint+test+tape-test), lint, test, tape-test, record.
`just` with no args lists all recipes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

justfile

@@ -0,0 +1,25 @@
+# systab task runner — `just` to list all recipes
+
+# List available recipes
+default:
+    @just --list
+
+# Run ShellCheck linter
+lint:
+    shellcheck systab
+
+# Run unit tests
+test:
+    ./test.sh
+
+# Run demo tape command tests (no VHS required)
+tape-test:
+    ./demo/test-tapes.sh
+
+# Run all checks: lint + unit tests + tape tests
+check: lint test tape-test
+
+# Record demo GIFs with VHS
+record:
+    vhs demo/quickstart.tape
+    vhs demo/all-features.tape