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_env.py

@@ -7,8 +7,8 @@ from pathlib import Path
 
 import pytest
 
-import castle_core.deploy as deploy
-from castle_core.registry import (
+import wildpc_core.deploy as deploy
+from wildpc_core.registry import (
     Deployment,
     NodeConfig,
     NodeRegistry,
@@ -23,7 +23,10 @@ def secret_env_dir(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> Path:
     d = tmp_path / "secrets" / "env"
     monkeypatch.setattr(deploy, "SECRET_ENV_DIR", d)
     monkeypatch.setattr(
-        deploy, "secret_env_path", lambda name: d / f"castle-{name}.service.env"
+        deploy,
+        "secret_env_path",
+        lambda name, kind="service": d
+        / f"wildpc-{name}{'-job' if kind == 'job' else ''}.service.env",
     )
     return d