feat(core): single-source data_dir/repos_dir via castle.yaml

The `castle` CLI and the `castle-api` service are two independent in-process
drivers of `castle_core`. Each resolved DATA_DIR/REPOS_DIR at import time from
its own process env (default /data/castle), persisted nowhere — so they silently
diverged, and `apply`/dashboard-apply crashed on a non-existent /data.

Make the loaded CastleConfig the single source of truth:
- Resolve data_dir/repos_dir only in load_config (env > castle.yaml > default),
  anchored to the config root; drop the DATA_DIR/REPOS_DIR module globals and the
  import-time file read entirely — no global twin that can disagree with the file.
- Thread config.data_dir/repos_dir through ensure_dirs, _env_context, tls_dir_for
  (now unified — deploy no longer inlines the tls path), and create/add/clone.
- ensure_dirs raises an actionable CastleDirError instead of a bare PermissionError;
  the api surfaces it as 422.
- doctor: "data dir writable" check + WARN when CASTLE_DATA_DIR/REPOS_DIR env
  overrides the file (the one remaining cross-process divergence vector).
- install.sh persists data_dir/repos_dir into castle.yaml (idempotent, non-default).
- Docs: registry.md globals + AGENTS.md roots.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

cli/src/castle_cli/commands/add.py

@@ -11,7 +11,7 @@ import argparse
 import tomllib
 from pathlib import Path
 
-from castle_cli.config import REPOS_DIR, load_config, save_config
+from castle_cli.config import load_config, save_config
 from castle_cli.manifest import BuildSpec, CommandsSpec, ProgramSpec
 
 
@@ -74,7 +74,7 @@ def run_add(args: argparse.Namespace) -> int:
         repo_url = target
         name = args.name or Path(target.rstrip("/")).name.removesuffix(".git")
         # Default local clone location; cloned later via `castle clone`.
-        source = str(REPOS_DIR / name)
+        source = str(config.repos_dir / name)
         src_path = Path(source)
     else:
         src_path = Path(target).expanduser().resolve()