refactor: Restructure ~/.castle/ as the instance directory

Move all instance state into ~/.castle/ with clear separation:
- code/        — user project source (was components/)
- artifacts/   — generated specs, built content (was generated/, static/)
- data/        — service runtime data (was /data/castle/)
- secrets/     — credentials

castle.yaml moves to ~/.castle/castle.yaml as the canonical location.
Source paths use repo: prefix for git repo programs (castle-api, app)
and relative paths for user projects (code/central-context).

Add idempotent install.sh for bootstrapping infrastructure (Docker,
Caddy, MQTT, Postgres, Neo4j) with interactive service detection.

Remove components/ from repo (now in ~/.castle/code/), deinit
submodules, remove .gitmodules.

core/tests/test_caddyfile.py

@@ -12,9 +12,9 @@ from castle_core.registry import DeployedComponent, NodeConfig, NodeRegistry
 
 
 @pytest.fixture(autouse=True)
-def _isolate_static_dir(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
-    """Use a temp dir for STATIC_DIR so tests don't depend on real ~/.castle."""
-    monkeypatch.setattr(caddyfile_mod, "STATIC_DIR", tmp_path / "static")
+def _isolate_content_dir(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
+    """Use a temp dir for CONTENT_DIR so tests don't depend on real ~/.castle."""
+    monkeypatch.setattr(caddyfile_mod, "CONTENT_DIR", tmp_path / "content")
 
 
 def _make_registry(