docs: add local code coverage instructions

README.md

@@ -153,7 +153,7 @@ bun dist/cli.js close test-session
 │   ├── types.ts               # Shared TypeScript types
 │   ├── core/
 │   │   ├── index.ts           # Core exports
-│   │   ├── launch.ts          # Launch session logic
+│   │   ├── open.ts            # Launch session logic
 │   │   ├── close.ts           # Close session logic
 │   │   └── utils.ts           # Helper functions
 │   └── tools/
@@ -166,6 +166,19 @@ bun dist/cli.js close test-session
 └── README.md
 ```
 
+## Local code coverage
+
+The test suite can generate a coverage report locally.
+
+```bash
+# Run tests with coverage (HTML report generated in ./coverage)
+ bun test --coverage
+# Open the HTML report
+ open coverage/index.html   # macOS
+```
+
+The `coverage` directory is ignored by Git but can be inspected to see which lines are exercised.
+
 ## License
 
 MIT