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_toolchains.py

@@ -6,8 +6,8 @@ from pathlib import Path
 
 import pytest
 
-from castle_core import toolchains
-from castle_core.toolchains import (
+from wildpc_core import toolchains
+from wildpc_core.toolchains import (
     ToolchainError,
     read_node_pin,
     resolve_node_bin,
@@ -26,7 +26,7 @@ def _install(root: Path, *versions: str) -> None:
 def nvm(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> Path:
     root = tmp_path / "nvm"
     root.mkdir()
-    monkeypatch.setenv("CASTLE_NODE_VERSIONS_DIR", str(root))
+    monkeypatch.setenv("WILDPC_NODE_VERSIONS_DIR", str(root))
     return root
 
 
@@ -117,5 +117,5 @@ class TestResolveNodeBin:
             resolve_node_bin(tmp_path)
 
     def test_default_dir_is_nvm(self, monkeypatch: pytest.MonkeyPatch) -> None:
-        monkeypatch.delenv("CASTLE_NODE_VERSIONS_DIR", raising=False)
+        monkeypatch.delenv("WILDPC_NODE_VERSIONS_DIR", raising=False)
         assert toolchains.node_versions_dir() == Path.home() / ".nvm" / "versions" / "node"