Refactor test cases for improved readability and consistency

- Updated test_dns.py to enhance mock function definitions and improve spacing for better readability.
- Modified test_filters.py to streamline assertions and ensure consistent formatting across test cases.
- Cleaned up test_import_export.py by organizing imports and ensuring consistent formatting in CSV and JSON tests.
- Improved test_main.py by refining mock setups and ensuring consistent error handling in assertions.

CONTRIBUTING.md

@@ -0,0 +1,117 @@
+# Contributing
+
+Thank you for improving `hosts`. Because the application modifies `/etc/hosts`
+with elevated privileges, correctness and honest user-facing behavior take
+priority over feature breadth.
+
+Use the [Forgejo issue tracker](https://git.s1q.dev/phg/hosts/issues) to report
+bugs or coordinate consequential changes.
+
+## Development setup
+
+The project requires Python 3.13 or newer and
+[uv](https://docs.astral.sh/uv/):
+
+```bash
+git clone https://git.s1q.dev/phg/hosts.git
+cd hosts
+uv sync
+uv run hosts
+```
+
+`uv run hosts` starts the real application and reads `/etc/hosts`. Stay in
+Read-only Mode unless you deliberately intend to exercise privileged behavior
+on your machine. Automated tests must use mocks and temporary files instead.
+
+## Code boundaries
+
+- `src/hosts/core/` owns Host Entries, parsing and serialization,
+  configuration, DNS resolution, undo/redo commands, and privileged file
+  operations.
+- `src/hosts/tui/` owns the Textual application, handlers, modals, widgets,
+  keybindings, and styles.
+- `tests/` mirrors both layers and isolates operating-system interactions.
+
+Keep domain and system logic out of Textual widgets. Route privileged writes
+through `HostsManager`; UI actions should coordinate services and render their
+results.
+
+Core code that is not reachable through the TUI is not yet a supported product
+feature. Do not advertise it in user-facing documentation until users can
+actually invoke it.
+
+## Domain language and decisions
+
+Read [CONTEXT.md](CONTEXT.md) before naming user-visible concepts. Use its
+canonical terms in the interface, documentation, issues, and tests where
+practical.
+
+Read the relevant architecture decision record before changing established
+contracts:
+
+- [ADR 0001](docs/adr/0001-privileged-mode-gates-writes.md) covers Privileged
+  Mode, sudo handling, system-file writes, and Pre-edit Backups.
+- [ADR 0002](docs/adr/0002-normalize-hosts-serialization.md) covers parsing,
+  serialization, comment placement, and formatting preservation.
+
+Add an ADR only for a consequential, surprising trade-off that would be
+expensive to reverse. Keep planned work in Forgejo issues rather than evergreen
+repository documents.
+
+## Safety contracts
+
+- The application starts in Read-only Mode.
+- Mutating actions require Privileged Mode, and a Pre-edit Backup must exist
+  before writes are enabled.
+- Default Entries are protected from mutation and must remain first when file
+  order changes.
+- All privileged writes go through `HostsManager`.
+- Serialization is semantic and normalized, not byte-preserving.
+- Tests never read or write the real `/etc/hosts`, invoke real `sudo`, or depend
+  on live DNS.
+
+Mock subprocess and resolver boundaries and inject a temporary Hosts File path.
+Treat any test capable of touching the real system file as a defect.
+
+## Make and validate a change
+
+Run the narrowest relevant test while developing. Examples:
+
+```bash
+uv run pytest tests/test_parser.py
+uv run pytest tests/test_manager.py
+uv run pytest tests/test_main.py
+```
+
+Before handing off any change, run the complete validation suite:
+
+```bash
+uv run pytest
+uv run ruff check .
+uv run ruff format --check .
+```
+
+Apply formatting with:
+
+```bash
+uv run ruff format .
+```
+
+Add or update tests for changed behavior. Test user-reachable workflows at the
+TUI boundary as well as isolated core behavior when both layers participate.
+
+## Keep documentation accurate
+
+Documentation is part of the behavior contract:
+
+- Update [README.md](README.md) when installation, major reachable features,
+  maturity, or prominent safety limitations change.
+- Update [the user guide](docs/user-guide.md) when workflows, shortcuts,
+  persistence, recovery, configuration, or troubleshooting change.
+- Update [CONTEXT.md](CONTEXT.md) when the canonical domain language changes;
+  keep it a glossary rather than an architecture guide or progress log.
+- Update ADR 0002 and the README if serialization stops being normalized or
+  becomes byte-preserving.
+
+Describe only current, reachable behavior in user-facing documentation. Record
+future work in an issue instead of presenting it as an available feature.