audit-D16: operator key must never be an agent identity

Background: CAYCE_OPS_OPERATOR_PUBKEY was set to Cayce nostr pubkey —
Cayce was approving itself. Signature was technically valid but semantically
null: human approval signed by the agent is not human approval.

- signing.py: validate_no_self_approval() rejects the operator pubkey when
  it matches (a) the pubkey derived from any private key in the config,
  or (b) any pubkey in CAYCE_OPS_FORBIDDEN_OPERATOR_PUBKEYS deny-list.
- executor/maild/publisher _signing() now calls the validation, so every
  daemon and every CLI approve-* path refuses to operate (D16).
- tests: 6 new (agent-key refusal, human key passes, empty passes,
  explicit denylist, end-to-end daemon refusal, derivation roundtrip).
- docs/DECISIONS.md: D16.

The operator signature is now produced OUTSIDE the VPS. CAYCE_OPS_OPERATOR_PUBKEY
must stay empty until the external signer exists — daemons refuse, which is
correct. The Cayce nsec itself is NOT touched (still the agent identity for
publishing and anchoring).

cayceops/signing.py

@@ -101,3 +101,72 @@ def verify_signature(
         )
     except (ValueError, TypeError):
         return False
+
+
+def pubkey_from_private_hex(private_key_hex: str) -> str:
+    """Derive the 64-char x-only public key from a 32-byte hex private key.
+
+    Raises ValueError on malformed input.
+    """
+    if not private_key_hex:
+        raise ValueError("empty private key")
+    try:
+        from coincurve import PublicKey
+
+        return PublicKey.from_secret(bytes.fromhex(private_key_hex)).format().hex()[2:]
+    except (ImportError, ValueError) as exc:
+        raise ValueError(f"cannot derive pubkey: {exc}") from exc
+
+
+class SelfApprovalError(RuntimeError):
+    """Raised when the configured operator pubkey is the identity of any
+    key held by the system itself (e.g. Cayce nostr identity). The design
+    requires a *separate human* operator key — never an agent key."""
+
+
+FORBIDDEN_IDENTITIES_ENV = "CAYCE_OPS_FORBIDDEN_OPERATOR_PUBKEYS"
+
+
+def forbidden_identities(cfg) -> list[str]:
+    """Pubkeys that may never act as the operator key.
+
+    Includes:
+      1. Any private key loaded from the config (nostr_private_key, or any
+         future agent identity).
+      2. An explicit deny-list in CAYCE_OPS_FORBIDDEN_OPERATOR_PUBKEYS
+         (comma-separated hex pubkeys) for keys held outside this process.
+    """
+    listed: list[str] = []
+    if cfg.nostr_private_key:
+        try:
+            listed.append(pubkey_from_private_hex(cfg.nostr_private_key))
+        except ValueError:
+            pass  # malformed nostr key: _signing() already catches it elsewhere
+    import os
+
+    raw = os.environ.get(FORBIDDEN_IDENTITIES_ENV, "")
+    for tok in raw.split(","):
+        tok = tok.strip().lower()
+        if len(tok) == 64:
+            listed.append(tok)
+    return listed
+
+
+def validate_no_self_approval(cfg) -> None:
+    """Refuse to operate if the operator pubkey equals any identity held by
+    the system. Raises SelfApprovalError.
+
+    Empty operator_pubkey is fine here (that's the 'not configured' state
+    which _signing() rejects elsewhere); this check is about *wrong* keys.
+    """
+    if not cfg.operator_pubkey:
+        return
+    me = cfg.operator_pubkey.strip().lower()
+    for forbidden in forbidden_identities(cfg):
+        if me == forbidden:
+            raise SelfApprovalError(
+                "CAYCE_OPS_OPERATOR_PUBKEY is an agent identity (Cayce or "
+                "another in-process key). The operator key must belong to a "
+                "human and must never reside on this server. Refusing to "
+                "operate. See docs/DECISIONS.md D16."
+            )