feat(local-identity): implement Stage 1 — core file store (NK-WP-0002-T01)

Deliverables:
- src/local_identity/gecos.py: /etc/passwd GECOS parsing, current_username()
- src/local_identity/user.py: UserRecord dataclass, ProductionIdentity, make_test_user()
  - Pure test-user derivation: <user>N / +testN email alias / source_user tracking
- src/local_identity/store.py: file store CRUD backed by LOCAL_IDENTITY_HOME
  - ~/.local-identity/ mode 700, user files mode 600
  - All path lookups dynamic (env-var override enables clean test isolation)
- src/local_identity/cli.py: init/list/show commands; email from flag > config > prompt
- pyproject.toml + uv.lock: pyyaml dep, local-identity script entry point

Tests (41 passing):
- test_gecos.py: 9 tests — simple/comma/empty/non-ASCII/whitespace GECOS, fallback
- test_user.py: 14 tests — test-user derivation, YAML roundtrip, non-ASCII, idempotency
- test_store.py: 18 tests — dir creation, permissions (700/600), CRUD, list, config,
  idempotency (reinit with --force produces identical users)

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

local-identity/tests/conftest.py

@@ -0,0 +1,19 @@
+"""
+Shared pytest fixtures for local-identity tests.
+
+The LOCAL_IDENTITY_HOME env var redirects the store to a temp directory,
+keeping tests isolated from the real ~/.local-identity store.
+"""
+
+import pytest
+
+
+@pytest.fixture
+def tmp_store(tmp_path, monkeypatch):
+    """
+    Redirect the local-identity store to a fresh temp directory.
+    Returns the Path to the store root (not yet created — call init_dirs() as needed).
+    """
+    store_path = tmp_path / ".local-identity"
+    monkeypatch.setenv("LOCAL_IDENTITY_HOME", str(store_path))
+    return store_path