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.

AGENTS.md

@@ -0,0 +1,80 @@
+# Repository guide
+
+`hosts` is a Python 3.13+ Textual application for inspecting and editing the
+system `/etc/hosts` file on macOS and Linux. Treat the implementation and tests
+as the source of truth; user-facing documentation describes only behavior that
+is reachable in the current application.
+
+## Context pointers
+
+- Read [CONTEXT.md](CONTEXT.md) when changing entry semantics, parsing, DNS
+  behavior, or user-facing terminology.
+- Read [ADR 0001](docs/adr/0001-privileged-mode-gates-writes.md) before changing
+  privileged mode, sudo handling, system-file writes, or backups.
+- Read [ADR 0002](docs/adr/0002-normalize-hosts-serialization.md) before changing
+  parsing, serialization, comment placement, or formatting preservation.
+
+## Commands
+
+```bash
+uv sync
+uv run hosts
+uv run pytest
+uv run pytest tests/test_parser.py
+uv run ruff check .
+uv run ruff format --check .
+uv run ruff format .
+```
+
+Use `uv run pytest <path>` for the narrowest relevant test during development.
+Before handing off a change, run the full test suite, Ruff lint, and Ruff format
+check.
+
+## Code boundaries
+
+- `src/hosts/core/` owns entries, parsing and serialization, configuration,
+  DNS resolution, filters, import/export, undo/redo commands, and privileged
+  file operations.
+- `src/hosts/tui/` owns the Textual application, handlers, modals, widgets,
+  keybindings, and styles.
+- `tests/` mirrors both layers. Keep system interactions behind mocks or
+  temporary files.
+
+Keep domain and system logic out of Textual widgets. Route privileged writes
+through `HostsManager`; UI actions should coordinate services and render their
+results.
+
+## Safety contracts
+
+- Tests must never read from or write to the real `/etc/hosts`, invoke real
+  `sudo`, or depend on live DNS. Inject temporary paths and mock subprocess and
+  resolver boundaries.
+- The application starts in read-only mode. Mutating actions require privileged
+  mode and a backup must exist before writes are enabled.
+- Default entries are protected from mutation and remain first when entries are
+  sorted.
+- Serialization is semantic and normalized, not byte-preserving. Update ADR
+  0002 and the README if that contract changes.
+- A core service is not a supported product feature until users can reach it
+  through the TUI.
+
+## Documentation discipline
+
+Keep `README.md` aligned with reachable behavior. Keep `CONTEXT.md` a glossary,
+not an architecture guide or progress log. Record an ADR only for a consequential,
+surprising trade-off that would be expensive to reverse. Track planned work in
+Forgejo issues rather than evergreen repository documents.
+
+## Agent skills
+
+### Issue tracker
+
+Issues are tracked in Forgejo using `fgj` against `git.s1q.dev/phg/hosts`. See `docs/agents/issue-tracker.md`.
+
+### Triage labels
+
+The repository uses the five default triage labels. See `docs/agents/triage-labels.md`.
+
+### Domain docs
+
+Domain documentation uses a single-context layout. See `docs/agents/domain.md`.