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_relations.py

@@ -1,18 +1,18 @@
-"""Tests for the relationship model (core/src/castle_core/relations.py)."""
+"""Tests for the relationship model (core/src/wildpc_core/relations.py)."""
 
 from __future__ import annotations
 
 import pytest
 
-import castle_core.config as C
-from castle_core import relations as R
-from castle_core.manifest import (
+import wildpc_core.config as C
+from wildpc_core import relations as R
+from wildpc_core.manifest import (
     CaddyDeployment,
     ProgramSpec,
     Requirement,
     SystemdDeployment,
 )
-from castle_core.stacks import tools_for
+from wildpc_core.stacks import tools_for
 
 
 def _dep(program: str, requires: list | None = None) -> SystemdDeployment:
@@ -32,8 +32,8 @@ def _static(program: str) -> CaddyDeployment:
     )
 
 
-def _cfg(programs: dict, deployments: dict) -> C.CastleConfig:
-    return C.CastleConfig(
+def _cfg(programs: dict, deployments: dict) -> C.WildpcConfig:
+    return C.WildpcConfig(
         root=None,
         gateway=C.GatewayConfig(port=9000),
         repo=None,
@@ -141,7 +141,7 @@ def test_stack_tool_missing_from_service_path_is_unmet(
 ) -> None:
     """The drift case: uv resolves in the caller's shell PATH but NOT on the
     service's curated runtime PATH → unmet *for the service*, even though a bare
-    `which` (what `castle tool list` uses) would report it present."""
+    `which` (what `wildpc tool list` uses) would report it present."""
     prog = ProgramSpec(id="svc", stack="python-fastapi")
     cfg = _cfg({"svc": prog}, {"svc": _dep("svc")})
     shell_only = "/home/me/.shell-tools"  # a dir the runtime PATH never includes