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/lib/labels.ts

@@ -82,7 +82,7 @@ export function detailPath(name: string, kind: string): string {
 
 /**
  * Full URL for a service exposed at <subdomain>.<gateway.domain>. The domain is
- * derived from the dashboard's own host (it is served at castle.<domain>), so
+ * derived from the dashboard's own host (it is served at wildpc.<domain>), so
  * this returns null when the dashboard is on a bare host (off mode, no subdomains).
  */
 export function subdomainUrl(subdomain: string): string | null {