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.

wildpc-api/tests/test_secrets_overrides.py

@@ -0,0 +1,33 @@
+"""HTTP-layer tests for the node-override endpoints (file-backend paths).
+
+The live OpenBao override round-trip is verified manually; here we pin the
+file-backend behavior (overrides are an OpenBao-only feature).
+"""
+
+from __future__ import annotations
+
+from fastapi.testclient import TestClient
+
+
+def test_list_overrides_empty_on_file_backend(client: TestClient) -> None:
+    r = client.get("/secrets/overrides")
+    assert r.status_code == 200
+    assert r.json() == {"overrides": {}}
+
+
+def test_set_override_rejected_on_file_backend(client: TestClient) -> None:
+    r = client.put("/secrets/overrides/primer/POSTGRES_PASSWORD", json={"value": "x"})
+    assert r.status_code == 400
+
+
+def test_get_missing_override_is_404(client: TestClient) -> None:
+    assert client.get("/secrets/overrides/primer/NOPE").status_code == 404
+
+
+def test_secrets_info_reports_backend(client: TestClient) -> None:
+    r = client.get("/secrets/info")
+    assert r.status_code == 200
+    body = r.json()
+    assert body["backend"] == "file"  # conftest forces file in tests
+    assert body["writable"] is True
+    assert "role" in body