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/src/components/GatewaySettings.tsx

@@ -4,7 +4,7 @@ import type { GatewayInfo } from "@/types"
 import { useSaveGatewayConfig } from "@/services/api/hooks"
 
 /** Editable gateway routing + public-exposure settings (domain / public_domain /
- *  tunnel / tls). Saves to castle.yaml; changes take effect on the next apply. */
+ *  tunnel / tls). Saves to wildpc.yaml; changes take effect on the next apply. */
 export function GatewaySettings({ gateway }: { gateway: GatewayInfo }) {
   const { mutate: save, isPending, data: saved } = useSaveGatewayConfig()
   const [editing, setEditing] = useState(false)
@@ -44,7 +44,7 @@ export function GatewaySettings({ gateway }: { gateway: GatewayInfo }) {
           <button onClick={() => setEditing(false)} className="flex items-center gap-1 text-xs px-2.5 py-1 rounded bg-[var(--border)] text-[var(--muted)]">
             <X size={12} /> Cancel
           </button>
-          <span className="text-xs text-[var(--muted)]">Run <span className="font-mono">castle apply</span> (or the Apply button) to converge.</span>
+          <span className="text-xs text-[var(--muted)]">Run <span className="font-mono">wildpc apply</span> (or the Apply button) to converge.</span>
         </div>
       </div>
     )