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/tests/test_stacks_api.py

@@ -0,0 +1,29 @@
+"""Tests for the stack-dependency endpoints (/stacks, /stacks/status, /stacks/{name})."""
+
+from fastapi.testclient import TestClient
+
+from wildpc_core.stacks import available_stacks
+
+
+class TestStacks:
+    def test_names_endpoint_stays_a_bare_list(self, client: TestClient) -> None:
+        """`GET /stacks` keeps its back-compat string[] shape (the create-form select
+        depends on it) even though the richer status lives at /stacks/status."""
+        resp = client.get("/stacks")
+        assert resp.status_code == 200
+        assert resp.json() == available_stacks()
+
+    def test_status_shape(self, client: TestClient) -> None:
+        resp = client.get("/stacks/status")
+        assert resp.status_code == 200
+        body = resp.json()
+        assert {s["name"] for s in body} == set(available_stacks())
+        st = next(s for s in body if s["name"] == "python-fastapi")
+        # python-fastapi declares uv; every tool carries its phase + fix.
+        assert {"in_use", "ok", "tools", "programs", "verbs"} <= st.keys()
+        uv = next(t for t in st["tools"] if t["command"] == "uv")
+        assert uv["phase"] == "both" and uv["install_hint"]
+
+    def test_detail_and_404(self, client: TestClient) -> None:
+        assert client.get("/stacks/python-cli").json()["name"] == "python-cli"
+        assert client.get("/stacks/nope").status_code == 404