Add castle secret CLI and fail-loud unresolved-secret apply gate

Writing a secret by hand meant knowing the active backend and its layout;
getting it wrong (a file write on an OpenBao fleet) left the value where the
resolver never reads it, so ${secret:NAME} silently degraded to the literal
<MISSING_SECRET:NAME> placeholder that a service then used as its credential.

- `castle secret {list|set|get|rm}` reads/writes the ACTIVE backend, so there's
  no wrong store to pick. `set NAME` with no value reads a hidden prompt / stdin.
- `castle apply` (and --plan) now refuses to converge any deployment whose
  ${secret:...} refs don't resolve in the active backend: exits non-zero, writes
  nothing, and names each deployment + secret + the `castle secret set` fix.
- New helpers in core/config.py: active_secret_backend(), active_backend_name(),
  secret_refs(). Gate impl: deploy._unresolved_secrets() + ApplyResult.blocked.
- Tests: test_deploy_secret_gate.py, TestSecretRefs, test_secret.py. Docs: AGENTS.md.

core/tests/test_config.py

@@ -12,6 +12,7 @@ from castle_core.config import (
     resolve_env_split,
     resolve_env_vars,
     save_config,
+    secret_refs,
 )
 from castle_core.manifest import ProgramSpec, SystemdDeployment
 
@@ -182,6 +183,22 @@ class TestResolveEnvVars:
         assert resolved["API_KEY"] == "<MISSING_SECRET:NONEXISTENT>"
 
 
+class TestSecretRefs:
+    """Tests for extracting ${secret:NAME} references (the apply-gate input)."""
+
+    def test_none_when_no_refs(self) -> None:
+        assert secret_refs("plain") == []
+        assert secret_refs("${port}/${data_dir}") == []
+
+    def test_single_and_composite(self) -> None:
+        assert secret_refs("${secret:API_KEY}") == ["API_KEY"]
+        assert secret_refs("neo4j/${secret:NEO4J_PW}") == ["NEO4J_PW"]
+
+    def test_multiple_deduped_in_order(self) -> None:
+        value = "${secret:A}:${secret:B}:${secret:A}"
+        assert secret_refs(value) == ["A", "B"]
+
+
 class TestResolveEnvSplit:
     """Tests for the secret/plain partition used to keep secrets out of units."""