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/src/wildpc_cli/config.py

@@ -0,0 +1,28 @@
+"""Re-export from wildpc-core for backward compatibility."""
+
+from wildpc_core.config import *  # noqa: F401, F403
+from wildpc_core.config import (  # noqa: F401 — explicit re-exports for type checkers
+    ARTIFACTS_DIR,
+    CODE_DIR,
+    CONTENT_DIR,
+    GENERATED_DIR,
+    SECRETS_DIR,
+    SPECS_DIR,
+    STATIC_DIR,
+    WILDPC_HOME,
+    GatewayConfig,
+    WildpcConfig,
+    ensure_dirs,
+    find_wildpc_root,
+    load_config,
+    resolve_env_vars,
+    save_config,
+)
+from wildpc_core.registry import (  # noqa: F401
+    REGISTRY_PATH,
+    Deployment,
+    NodeConfig,
+    NodeRegistry,
+    load_registry,
+    save_registry,
+)