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>

AGENTS.md

@@ -247,8 +247,12 @@ defaults:
 | `${secret:NAME}` | contents of `~/.castle/secrets/NAME` (mode 700) |
 
 **Never** put secrets in `castle.yaml` or project dirs — use `${secret:…}`.
-Roots: **`CASTLE_HOME`** (config/code/artifacts/secrets, default `~/.castle`) and
-**`CASTLE_DATA_DIR`** (program data, default `/data/castle`) — both env-overridable.
+Roots: **`CASTLE_HOME`** (config/code/artifacts/secrets, default `~/.castle`,
+env-only — it *contains* castle.yaml) and **program data** (base of `${data_dir}`,
+default `/data/castle`) + **repos** (default `/data/repos`). The latter two resolve
+**env > `castle.yaml` > default** — set `data_dir:` / `repos_dir:` in `castle.yaml`
+(the single source of truth both the CLI and the api read), not a per-shell env var
+that only one of them sees. → **`docs/registry.md`** (castle.yaml globals).
 
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