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.

castle-api/tests/conftest.py

@@ -86,7 +86,7 @@ def registry_path(tmp_path: Path, castle_root: Path) -> Generator[Path, None, No
                 run_cmd=["uv", "run", "test-svc"],
                 env={
                     "TEST_SVC_PORT": "19000",
-                    "TEST_SVC_DATA_DIR": "/data/castle/test-svc",
+                    "TEST_SVC_DATA_DIR": "/home/user/.castle/data/test-svc",
                 },
                 description="Test service",
                 behavior="daemon",