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>

src/planemapper/provisioning/config.py

@@ -1 +1,22 @@
-# stub
+import json
+from pathlib import Path
+
+from planemapper.constants import CONFIG_PATH as _CONFIG_PATH
+
+# Module-level reference so tests can monkeypatch this name
+CONFIG_PATH: Path = _CONFIG_PATH
+
+
+def read() -> dict:
+    with CONFIG_PATH.open() as f:
+        return json.load(f)
+
+
+def write(data: dict) -> None:
+    CONFIG_PATH.parent.mkdir(parents=True, exist_ok=True)
+    with CONFIG_PATH.open("w") as f:
+        json.dump(data, f, indent=2)
+
+
+def wipe() -> None:
+    CONFIG_PATH.unlink(missing_ok=True)