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.

app/.env

@@ -1,4 +1,4 @@
 # The API base URL is derived at runtime (see src/services/api/client.ts):
-#   - dashboard at castle-app.<domain>  -> castle-api.<domain>  (cross-origin, CORS)
+#   - dashboard at wildpc-app.<domain>  -> wildpc-api.<domain>  (cross-origin, CORS)
 #   - dev / off-mode :9000 (bare host)  -> /api                 (same-origin)
 # Set VITE_API_BASE_URL only to force a specific base (rarely needed).