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/src/local_identity/__init__.py

@@ -0,0 +1,2 @@
+# local-identity: zero-dependency bootstrap user store for net-kingdom environments.
+# See docs/LocalIdentity.md for design, scope, and security model.