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/AddProgramForm.tsx

@@ -21,7 +21,7 @@ function splitPath(raw: string): { dir: string | null; filter: string } {
   return { dir: raw.slice(0, idx), filter: raw.slice(idx + 1) }
 }
 
-/** Adopt an existing repo as a program (the web `castle program add`). Programs
+/** Adopt an existing repo as a program (the web `wildpc program add`). Programs
  * live on the *server's* filesystem, so the picker browses the server's dirs: type
  * a path (autocompletes as you go), navigate the listing, or paste a git URL. */
 export function AddProgramForm({
@@ -181,7 +181,7 @@ export function AddProgramForm({
       {target && (
         <p className="text-xs text-[var(--muted)]">
           Registering <span className="font-mono text-[var(--foreground)]">{target}</span>
-          {isGit && " (cloned later via castle clone)"}
+          {isGit && " (cloned later via wildpc clone)"}
         </p>
       )}