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/tests/test_create.py

@@ -18,9 +18,9 @@ class TestCreateCommand:
         with (
             patch("castle_cli.commands.create.load_config") as mock_load,
             patch("castle_cli.commands.create.save_config") as mock_save,
-            patch("castle_cli.commands.create.REPOS_DIR", repos),
         ):
             config = load_config(castle_root)
+            config.repos_dir = repos
             mock_load.return_value = config
 
             from castle_cli.commands.create import run_create
@@ -57,9 +57,9 @@ class TestCreateCommand:
         with (
             patch("castle_cli.commands.create.load_config") as mock_load,
             patch("castle_cli.commands.create.save_config"),
-            patch("castle_cli.commands.create.REPOS_DIR", repos),
         ):
             config = load_config(castle_root)
+            config.repos_dir = repos
             mock_load.return_value = config
 
             from castle_cli.commands.create import run_create
@@ -84,9 +84,9 @@ class TestCreateCommand:
         with (
             patch("castle_cli.commands.create.load_config") as mock_load,
             patch("castle_cli.commands.create.save_config"),
-            patch("castle_cli.commands.create.REPOS_DIR", repos),
         ):
             config = load_config(castle_root)
+            config.repos_dir = repos
             mock_load.return_value = config
 
             from castle_cli.commands.create import run_create
@@ -138,9 +138,9 @@ class TestCreateCommand:
         with (
             patch("castle_cli.commands.create.load_config") as mock_load,
             patch("castle_cli.commands.create.save_config"),
-            patch("castle_cli.commands.create.REPOS_DIR", tmp_path / "repos"),
         ):
             config = load_config(castle_root)
+            config.repos_dir = tmp_path / "repos"
             mock_load.return_value = config
 
             from castle_cli.commands.create import run_create