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_mesh_config_api.py

@@ -0,0 +1,26 @@
+"""HTTP-layer tests for the /mesh/config endpoints (mesh-disabled paths).
+
+The write/read-through-the-client behavior is covered by test_nats_integration;
+here we pin the endpoint wiring when no mesh client is attached.
+"""
+
+from __future__ import annotations
+
+from fastapi.testclient import TestClient
+
+
+def test_list_config_reports_role_when_mesh_disabled(client: TestClient) -> None:
+    r = client.get("/mesh/config")
+    assert r.status_code == 200
+    body = r.json()
+    assert body["keys"] == []
+    assert body["role"] == "follower"  # test-node has no explicit role
+
+
+def test_get_missing_config_is_404(client: TestClient) -> None:
+    assert client.get("/mesh/config/does/not/exist").status_code == 404
+
+
+def test_write_without_mesh_is_503(client: TestClient) -> None:
+    r = client.put("/mesh/config/fleet/motd", json={"value": "x"})
+    assert r.status_code == 503