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>

castle-api/src/castle_api/deploy_routes.py

@@ -11,6 +11,7 @@ from __future__ import annotations
 from fastapi import APIRouter, HTTPException
 from pydantic import BaseModel
 
+from castle_core.config import CastleDirError
 from castle_core.deploy import apply
 
 router = APIRouter(tags=["apply"])
@@ -52,6 +53,10 @@ def run_apply(request: ApplyRequest | None = None) -> ApplyResponse:
         result = apply(target_name=name, plan=plan)
     except FileNotFoundError as e:
         raise HTTPException(status_code=404, detail=str(e)) from e
+    except CastleDirError as e:
+        # A fixable misconfiguration (e.g. data_dir points somewhere unwritable), not a
+        # server fault — return the actionable message so the dashboard can show it.
+        raise HTTPException(status_code=422, detail=str(e)) from e
     except Exception as e:
         raise HTTPException(status_code=500, detail=str(e)) from e