Fix screenshot affecting terminal state in open sessions

- Add get_screen_snapshot() method that doesn't mutate terminal state
- Use change counter for reliable activity detection instead of dirty flag
- Update screenshot handler to use non-mutating snapshot method
- Refactor tests to use shared fixtures and reduce duplication
- Update copilot-instructions.md with detailed Makefile usage

.github/copilot-instructions.md

@@ -1,4 +1,51 @@
-Use the Makefile to run linting, testing, and builds.
-Aim for good test coverage.
-Review tests periodically with a view to consolidate/parameterize and remove redundancy.
-Debug issues systematically. Search for and review documentation as needed.
+# Copilot Development Instructions
+
+## Makefile Usage (MANDATORY)
+
+Always use the Makefile for development tasks. Never run raw `pytest`, `ruff`, or `bun` commands directly.
+
+### Quick Reference
+
+| Task | Command | Description |
+|------|---------|-------------|
+| Run tests | `make test` | Run pytest |
+| Run linter | `make lint` | Run ruff linter |
+| Format code | `make format` | Auto-format with ruff |
+| Full check | `make check` | Run lint + coverage |
+| Coverage report | `make coverage` | Run pytest with coverage |
+| Build frontend | `make build` | TypeScript typecheck + bundle |
+| Quick frontend build | `make build-fast` | Bundle without typecheck |
+| Watch mode | `make bundle-watch` | Frontend dev with auto-rebuild |
+| Install dev deps | `make install-dev` | Install package + dev dependencies |
+| Clean build | `make build-all` | Full reproducible build from scratch |
+| See all targets | `make help` | Show all available commands |
+
+### Development Workflow
+
+1. **Before making changes**: Run `make check` to establish baseline
+2. **After making changes**: Run `make check` to verify no regressions
+3. **Frontend changes**: Use `make build` or `make build-fast`
+4. **Full rebuild**: Use `make build-all` (cleans everything first)
+
+### Clean Targets
+
+- `make clean` - Remove Python cache files only
+- `make bundle-clean` - Remove frontend build artifacts (node_modules, etc.)
+- `make clean-all` - Remove everything
+
+### Version Management
+
+- `make bump-patch` - Bump patch version and create git tag
+
+## Testing Guidelines
+
+- Aim for good test coverage
+- Review tests periodically to consolidate/parameterize and remove redundancy
+- Use fixtures from `tests/conftest.py` instead of duplicating setup code
+- Prefer parameterized tests for similar test cases
+
+## Code Style
+
+- Do not use heredocs or random shell commands
+- Prefer `make` and ecosystem tools (pip, bun) over manual operations
+- Debug issues systematically - search for and review documentation as needed