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_tool_schema.py

@@ -1,4 +1,4 @@
-"""Tests for castle_core.tool_schema — deriving neutral tool-call cores from --help."""
+"""Tests for wildpc_core.tool_schema — deriving neutral tool-call cores from --help."""
 
 from __future__ import annotations
 
@@ -6,7 +6,7 @@ from types import SimpleNamespace
 
 import pytest
 
-from castle_core.tool_schema import (
+from wildpc_core.tool_schema import (
     ToolSchemaError,
     _command_core,
     _extract_subcommands,
@@ -115,7 +115,7 @@ class TestDerive:
 
 
 class TestLLMAssistHelpers:
-    """Deterministic helpers that feed / validate the (castle-api) LLM assist."""
+    """Deterministic helpers that feed / validate the (wildpc-api) LLM assist."""
 
     def test_collect_tool_help_real_tool(self) -> None:
         cfg = _fake_config({"python3": SimpleNamespace(source=None)})