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.

wildpc-api/pyproject.toml

@@ -0,0 +1,38 @@
+[project]
+name = "wildpc-api"
+version = "0.1.0"
+description = "Wild PC API"
+requires-python = ">=3.13"
+dependencies = [
+    "fastapi>=0.115.0",
+    "uvicorn[standard]>=0.34.0",
+    "pydantic-settings>=2.0.0",
+    "httpx>=0.27.0",
+    "wildpc-core",
+    "nats-py>=2.9.0",
+    "zeroconf>=0.131.0",
+]
+
+[tool.uv.sources]
+wildpc-core = { workspace = true }
+
+[project.scripts]
+wildpc-api = "wildpc_api.main:run"
+
+[build-system]
+requires = ["hatchling"]
+build-backend = "hatchling.build"
+
+[tool.hatch.build.targets.wheel]
+packages = ["src/wildpc_api"]
+
+[dependency-groups]
+dev = [
+    "pytest>=7.0.0",
+    "pytest-asyncio>=0.23.0",
+    "httpx>=0.27.0",
+    "pyyaml>=6.0.0",
+]
+
+[tool.ruff.lint.isort]
+known-first-party = ["wildpc_api"]