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.

bootstrap/deployments/services/wildpc-api.yaml

@@ -1,9 +1,9 @@
-program: castle-api
-description: Castle API
+program: wildpc-api
+description: Wild PC API
 manager: systemd
 run:
   launcher: python
-  program: castle-api
+  program: wildpc-api
 expose:
   http:
     internal:
@@ -14,5 +14,5 @@ manage:
   systemd: {}
 defaults:
   env:
-    CASTLE_API_PORT: ${port}
-    CASTLE_API_DATA_DIR: ${data_dir}
+    WILDPC_API_PORT: ${port}
+    WILDPC_API_DATA_DIR: ${data_dir}

bootstrap/deployments/services/wildpc-gateway.yaml

@@ -1,4 +1,4 @@
-# The Caddy gateway. `__SPECS_DIR__` is substituted with $CASTLE_HOME/artifacts/specs
+# The Caddy gateway. `__SPECS_DIR__` is substituted with $WILDPC_HOME/artifacts/specs
 # at install time (install.sh seed_control_plane). For acme TLS, add a
 # CLOUDFLARE_API_TOKEN to defaults.env later — see docs/dns-and-tls.md.
 description: Caddy reverse proxy gateway