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.

cli/tests/test_secret.py

@@ -1,4 +1,4 @@
-"""Tests for the `castle secret` command (reads/writes the active backend)."""
+"""Tests for the `wildpc secret` command (reads/writes the active backend)."""
 
 from __future__ import annotations
 
@@ -13,13 +13,13 @@ def file_secrets(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> Path:
     """Force the active backend to a temp file store."""
     secrets = tmp_path / "secrets"
     secrets.mkdir()
-    monkeypatch.setenv("CASTLE_SECRET_BACKEND", "file")
-    monkeypatch.setattr("castle_core.config.SECRETS_DIR", secrets)
+    monkeypatch.setenv("WILDPC_SECRET_BACKEND", "file")
+    monkeypatch.setattr("wildpc_core.config.SECRETS_DIR", secrets)
     return secrets
 
 
 def _run(**kw: object) -> int:
-    from castle_cli.commands.secret import run_secret
+    from wildpc_cli.commands.secret import run_secret
 
     return run_secret(Namespace(**kw))