feat(secrets): pluggable secret backend with OpenBao read (Phase 4)

- secret_backends.py: SecretBackend protocol, FileSecretBackend (historical),
  OpenBaoBackend (KV-v2 read + file fallback), build_backend() env-selected
- _read_secret delegates to the active backend; default is file, so production
  is unchanged until CASTLE_SECRET_BACKEND=openbao
- OpenBao token bootstraps from the file backend; missing/unreachable falls back
- tests: file hit/miss, backend selection, unreachable + empty-token fallback

Read path verified live against castle-openbao. Write path, auto-unseal-on-boot,
and TLS hardening documented as remaining for full OpenBao production use.

core/tests/test_secret_backends.py

@@ -0,0 +1,54 @@
+"""Tests for the pluggable secret backends (file default, OpenBao opt-in)."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from castle_core.secret_backends import (
+    FileSecretBackend,
+    OpenBaoBackend,
+    build_backend,
+)
+
+
+def test_file_backend_read_hit(tmp_path: Path) -> None:
+    (tmp_path / "MY_SECRET").write_text("value\n")
+    assert FileSecretBackend(tmp_path).read("MY_SECRET") == "value"
+
+
+def test_file_backend_read_miss(tmp_path: Path) -> None:
+    assert FileSecretBackend(tmp_path).read("ABSENT") is None
+
+
+def test_build_backend_defaults_to_file(tmp_path: Path, monkeypatch) -> None:
+    monkeypatch.delenv("CASTLE_SECRET_BACKEND", raising=False)
+    assert isinstance(build_backend(tmp_path), FileSecretBackend)
+
+
+def test_build_backend_openbao_selected(tmp_path: Path, monkeypatch) -> None:
+    monkeypatch.setenv("CASTLE_SECRET_BACKEND", "openbao")
+    assert isinstance(build_backend(tmp_path), OpenBaoBackend)
+
+
+def test_openbao_falls_back_to_file_when_unreachable(tmp_path: Path) -> None:
+    """An unreachable vault (or empty token) resolves via the file fallback."""
+    (tmp_path / "ONLY_IN_FILE").write_text("from-file")
+    backend = OpenBaoBackend(
+        addr="http://127.0.0.1:1",  # nothing listening
+        token="dummy",
+        mount="castle",
+        fallback=FileSecretBackend(tmp_path),
+    )
+    assert backend.read("ONLY_IN_FILE") == "from-file"
+    assert backend.read("NOT_ANYWHERE") is None
+
+
+def test_openbao_empty_token_uses_fallback(tmp_path: Path) -> None:
+    (tmp_path / "K").write_text("v")
+    backend = OpenBaoBackend(
+        addr="http://127.0.0.1:8200",
+        token="",  # no token → never hits the network
+        mount="castle",
+        fallback=FileSecretBackend(tmp_path),
+    )
+    assert backend.read("K") == "v"