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-gateway.yaml

@@ -0,0 +1,22 @@
+# 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
+manager: systemd
+run:
+  launcher: command
+  argv:
+  - caddy
+  - run
+  - --config
+  - __SPECS_DIR__/Caddyfile
+  - --adapter
+  - caddyfile
+expose:
+  http:
+    internal:
+      port: 9000
+    health_path: /
+manage:
+  systemd:
+    exec_reload: caddy reload --config __SPECS_DIR__/Caddyfile --adapter caddyfile