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.

core/tests/test_dns.py

@@ -2,8 +2,8 @@
 
 from __future__ import annotations
 
-import castle_core.generators.dns as dns
-from castle_core.generators.dns import _zone_for, reconcile_public_dns
+import wildpc_core.generators.dns as dns
+from wildpc_core.generators.dns import _zone_for, reconcile_public_dns
 
 TID = "tid-abc"
 TARGET = f"{TID}.cfargotunnel.com"
@@ -18,7 +18,7 @@ class _FakeCloudflare:
     """A minimal fake of the Cloudflare API used by reconcile_public_dns.
 
     ``records`` maps zone_id -> {name: (record_id, content)}. Records POST/DELETE
-    calls so tests can assert exactly what castle created/removed.
+    calls so tests can assert exactly what wildpc created/removed.
     """
 
     def __init__(self, records: dict[str, dict[str, tuple[str, str]]]):
@@ -75,11 +75,11 @@ def test_creates_apex_and_subdomain_in_correct_zones(monkeypatch) -> None:
 
 
 def test_deletes_managed_cname_no_longer_desired(monkeypatch) -> None:
-    # z_payne has a stale castle-managed CNAME + a hand-managed one pointing elsewhere.
+    # z_payne has a stale wildpc-managed CNAME + a hand-managed one pointing elsewhere.
     fake = _FakeCloudflare(records={
         "z_payne": {
-            "payne.io": ("r1", TARGET),          # castle-managed, still desired
-            "old.payne.io": ("r2", TARGET),      # castle-managed, now stale → delete
+            "payne.io": ("r1", TARGET),          # wildpc-managed, still desired
+            "old.payne.io": ("r2", TARGET),      # wildpc-managed, now stale → delete
             "keep.payne.io": ("r3", "other.example.com"),  # NOT ours → never touched
         },
         "z_ex": {},