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/detail/ServiceFields.tsx

@@ -201,7 +201,7 @@ export function ServiceFields({ service, onSave, onDelete }: Props) {
             width="w-32"
             mono
             placeholder="9001"
-            hint="The port the service listens on. Castle health-checks and proxies this port; map it to the program's own var with ${port} in Environment."
+            hint="The port the service listens on. Wild PC health-checks and proxies this port; map it to the program's own var with ${port} in Environment."
           />
           <TextField
             label="Health path"
@@ -210,7 +210,7 @@ export function ServiceFields({ service, onSave, onDelete }: Props) {
             width="w-48"
             mono
             placeholder="/health"
-            hint="HTTP path castle polls to report up/down."
+            hint="HTTP path wildpc polls to report up/down."
           />
           <Field
             label="Reach"
@@ -263,7 +263,7 @@ export function ServiceFields({ service, onSave, onDelete }: Props) {
         onSave={handleSave}
         onDelete={onDelete ? () => onDelete(service.id) : undefined}
         deleteLabel="Remove service"
-        confirmMessage={`Remove service "${service.id}" from castle.yaml? Run a deploy afterward to tear down its unit.`}
+        confirmMessage={`Remove service "${service.id}" from wildpc.yaml? Run a deploy afterward to tear down its unit.`}
       />
     </div>
   )