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>

README.md

@@ -239,6 +239,15 @@ After cloning, enable the pre-commit hook (runs ShellCheck + tests):
 git config core.hooksPath .githooks
 ```
 
+Common tasks via [`just`](https://github.com/casey/just):
+
+```bash
+just check    # lint + unit tests + tape tests
+just test     # unit tests only
+just lint     # ShellCheck only
+just record   # re-record demo GIFs with VHS
+```
+
 
 ## FAQ
 

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