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_caddyfile_remote.py

@@ -5,13 +5,13 @@ from __future__ import annotations
 from pathlib import Path
 
 import yaml
-from castle_core.config import load_config
-from castle_core.generators.caddyfile import compute_routes
-from castle_core.registry import Deployment, NodeConfig, NodeRegistry
+from wildpc_core.config import load_config
+from wildpc_core.generators.caddyfile import compute_routes
+from wildpc_core.registry import Deployment, NodeConfig, NodeRegistry
 
 
 def _config_requiring_widget(root: Path) -> None:
-    (root / "castle.yaml").write_text(yaml.safe_dump({"gateway": {"port": 18000}}))
+    (root / "wildpc.yaml").write_text(yaml.safe_dump({"gateway": {"port": 18000}}))
     svc_dir = root / "deployments" / "services"
     svc_dir.mkdir(parents=True)
     (svc_dir / "consumer.yaml").write_text(
@@ -79,7 +79,7 @@ def test_breaker_no_route_when_peer_absent(tmp_path: Path) -> None:
 
 def test_no_remote_route_for_unconsumed_service(tmp_path: Path) -> None:
     """A peer service nobody requires is not routed."""
-    (tmp_path / "castle.yaml").write_text(yaml.safe_dump({"gateway": {"port": 18000}}))
+    (tmp_path / "wildpc.yaml").write_text(yaml.safe_dump({"gateway": {"port": 18000}}))
     config = load_config(tmp_path)  # no requires anywhere
     routes = compute_routes(
         _local(), config, {"tower": _peer_with_widget("10.0.0.5")}