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/pyproject.toml

@@ -1,7 +1,7 @@
 [project]
-name = "castle-core"
+name = "wildpc-core"
 version = "0.1.0"
-description = "Castle platform core library - manifest models, config, and generators"
+description = "Wild PC platform core library - manifest models, config, and generators"
 requires-python = ">=3.11"
 dependencies = [
     "pyyaml>=6.0.0",
@@ -14,7 +14,7 @@ requires = ["hatchling"]
 build-backend = "hatchling.build"
 
 [tool.hatch.build.targets.wheel]
-packages = ["src/castle_core"]
+packages = ["src/wildpc_core"]
 
 [dependency-groups]
 dev = [
@@ -23,4 +23,4 @@ dev = [
 ]
 
 [tool.ruff.lint.isort]
-known-first-party = ["castle_core"]
+known-first-party = ["wildpc_core"]