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/config.py

@@ -3,8 +3,13 @@
 from castle_core.config import *  # noqa: F401, F403
 from castle_core.config import (  # noqa: F401 — explicit re-exports for type checkers
     CASTLE_HOME,
-    GENERATED_DIR,
+    CODE_DIR,
+    ARTIFACTS_DIR,
+    SPECS_DIR,
+    CONTENT_DIR,
+    DATA_DIR,
     SECRETS_DIR,
+    GENERATED_DIR,
     STATIC_DIR,
     CastleConfig,
     GatewayConfig,