Improve check output and start adding E2E testing

Author: zph

TESTING.md

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+# Testing
+
+## CLI
+
+| Command | Description                                                | Unit Tests | Manual Testing Required               |
+| ------- | ---------------------------------------------------------- | ---------- | ------------------------------------- |
+| check   | Check language validity and formatting of a notebook       | ✅         | No                                    |
+| convert | Convert an existing runbook to different format            | ✅         | Yes - Test conversion between formats |
+| create  | Create a new runbook from [template]                       | ✅         | Yes - Verify template rendering       |
+| diff    | Diff two notebooks                                         | ✅         | Yes - Visual verification             |
+| edit    | Edit an existing runbook                                   | ✅         | Yes - Interactive editing             |
+| init    | Initialize a folder as a runbook repository                | ✅         | No                                    |
+| list    | List runbooks                                              | ✅         | No                                    |
+| plan    | Prepares the runbook for execution by injecting parameters | ❌         | Yes - Parameter injection             |
+| review  | [Unimplemented] Entrypoint for reviewing runbook           | ❌         | N/A                                   |
+| run     | Run a notebook                                             | ✅         | Yes - Full execution                  |
+| show    | Show runbook parameters and metadata                       | ✅         | No                                    |
+| version | Display version information about runbook                  | ✅         | No                                    |
+
+Commands marked with "Manual Testing Required: Yes" should be tested manually as
+they cannot be easily automation tested. Key areas to verify:
+
+- `convert`: Test conversion between all supported formats
+- `create`: Verify template rendering and parameter substitution
+- `diff`: Visually verify diff output is correct
+- `edit`: Test interactive editing workflow
+- `plan`: Verify parameter injection and validation
+- `run`: Test full execution with various parameter combinations