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.

cli/src/castle_cli/commands/gateway.py

@@ -5,7 +5,7 @@ from __future__ import annotations
 import argparse
 import subprocess
 
-from castle_core.config import GENERATED_DIR
+from castle_core.config import SPECS_DIR
 from castle_core.generators.caddyfile import generate_caddyfile_from_registry
 from castle_core.registry import REGISTRY_PATH, load_registry
 
@@ -24,7 +24,7 @@ def _write_generated_files() -> None:
         return
 
     registry = load_registry()
-    caddyfile_path = GENERATED_DIR / "Caddyfile"
+    caddyfile_path = SPECS_DIR / "Caddyfile"
     caddyfile_path.write_text(generate_caddyfile_from_registry(registry))
     print(f"  Generated {caddyfile_path}")