Rename Castle -> Wild PC across the repo

Repo-side rename only (Phases 1-3 of the migration plan); the live box
(~/.castle, systemd units, /data/castle, domains) is a separate cutover.

- Slug `castle` -> `wildpc`: CLI command, module names (wildpc_core/cli/api),
  dist names, entry point `wildpc = wildpc_cli.main:main`.
- Identifiers: CastleConfig/NATSClient/DirError/MDNS -> Wildpc*.
- Env/constants: CASTLE_* -> WILDPC_*; ~/.castle -> ~/.wildpc, castle.yaml ->
  wildpc.yaml, /data/castle -> /data/wildpc.
- Systemd UNIT_PREFIX castle- -> wildpc-; own programs castle-api/gateway/etc.
- Display prose "Castle" -> "Wild PC" in docs, agent-guide files, README, frontend.
- Package dirs and bootstrap yaml renamed via git mv; lockfiles regenerated;
  redundant nested uv.lock files dropped (workspace root lock is authoritative).

Tests: core 273, cli 47, wildpc-api 120 all pass. Frontend type-checks + builds.
Fixed a stale test fixture (secret_env_path kind arg) broken pre-rename.

core/tests/test_deploy_secret_gate.py

@@ -1,6 +1,6 @@
 """Tests for the apply-time unresolved-secret gate.
 
-`castle apply` must refuse to converge a deployment whose ``${secret:NAME}`` env
+`wildpc apply` must refuse to converge a deployment whose ``${secret:NAME}`` env
 references the active backend can't resolve — otherwise the value silently
 degrades to a ``<MISSING_SECRET:NAME>`` placeholder and the service starts with a
 bogus credential (the immich-DB-password-to-the-wrong-backend failure).
@@ -13,7 +13,7 @@ from types import SimpleNamespace
 
 import pytest
 
-from castle_core.deploy import _unresolved_secrets
+from wildpc_core.deploy import _unresolved_secrets
 
 
 def _spec(env: dict[str, str], enabled: bool = True) -> SimpleNamespace:
@@ -26,8 +26,8 @@ def file_backend(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> Path:
     secrets = tmp_path / "secrets"
     secrets.mkdir()
     (secrets / "PRESENT").write_text("value\n")
-    monkeypatch.setenv("CASTLE_SECRET_BACKEND", "file")
-    monkeypatch.setattr("castle_core.config.SECRETS_DIR", secrets)
+    monkeypatch.setenv("WILDPC_SECRET_BACKEND", "file")
+    monkeypatch.setattr("wildpc_core.config.SECRETS_DIR", secrets)
     return secrets