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/src/castle_core/registry.py

@@ -8,10 +8,10 @@ from pathlib import Path
 
 import yaml
 
-from castle_core.config import CASTLE_HOME
+from castle_core.config import CONTENT_DIR, SPECS_DIR
 
-REGISTRY_PATH = CASTLE_HOME / "registry.yaml"
-STATIC_DIR = CASTLE_HOME / "static"
+REGISTRY_PATH = SPECS_DIR / "registry.yaml"
+STATIC_DIR = CONTENT_DIR  # backwards-compat alias
 
 
 @dataclass