Implement story 1.2: config read/write/wipe

Add provisioning/config.py with read/write/wipe functions, update
conftest.py with autouse CONFIG_PATH patch fixture, write 7 tests
covering all acceptance criteria, and extend pyproject.toml
per-file-ignores so conftest.py may import from provisioning.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/conftest.py

@@ -2,7 +2,9 @@ from pathlib import Path
 
 import pytest
 
+import planemapper.provisioning.config as config_module
 
-@pytest.fixture
-def tmp_config_path(tmp_path: Path) -> Path:
-    return tmp_path / "config.json"
+
+@pytest.fixture(autouse=True)
+def patch_config_path(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setattr(config_module, "CONFIG_PATH", tmp_path / "config.json")