audit-1 (critical 1): mandatory operator signatures bound to content+deployment

- signing module: canonical content hashes + approval_message
  sha256(cayce-ops:<type>:<deployment_id>:<id>:<content_hash>)
- approve/approve-draft/publisher-approve REQUIRE signature; without
  CAYCE_OPS_OPERATOR_PUBKEY+DEPLOYMENT_ID the daemons refuse to
  execute/send/publish (no degraded mode)
- run-time re-verification at execute/send/publish: unsigned rows (raw SQL
  approval) or edited content revert to pre-approval state, audited as error
- FULL signatures stored on rows (operator_signature, migration 0003) and
  in audit detail — never truncated
- signature failures audited from CLI wrappers too (1f)
- AUTONOMOUS_MODE approval path removed (D13): no non-human approvals

tests: SQL-approved row refused at execute/send/publish; to_addr change
blocks send; foreign deployment/content signatures refused; full-signature
persistence. 65 tests green.

cayceops/signing.py

@@ -0,0 +1,103 @@
+"""Operator approval signing — mandatory, content- and deployment-bound.
+
+Every human approval (executor approve / maild approve-draft / publisher
+approve) must carry a BIP-340 signature made with the operator key:
+
+    message = sha256("cayce-ops:<type>:<deployment_id>:<id>:<content_hash>")
+
+where content_hash binds the approval to the exact approved content:
+
+    proposal: canonical json of {kind, title, url, detail}
+    mail:     canonical json of {to_addr, subject, body}
+    content:  canonical json of {channel, caption, link}
+
+There is NO degraded mode: without CAYCE_OPS_OPERATOR_PUBKEY and
+CAYCE_OPS_DEPLOYMENT_ID the daemons refuse to execute/send/publish
+(see cayceops.config.Config.signing_credentials).
+"""
+
+from __future__ import annotations
+
+import hashlib
+import json
+
+
+class SignatureError(RuntimeError):
+    """Raised when an approval signature is missing, invalid, or foreign."""
+
+
+def _canonical(obj: dict) -> str:
+    return json.dumps(obj, sort_keys=True, separators=(",", ":"), ensure_ascii=False)
+
+
+def _sha256_hex(text: str) -> str:
+    return hashlib.sha256(text.encode("utf-8")).hexdigest()
+
+
+def proposal_content_hash(kind: str, title: str, url: str | None, detail: dict | None) -> str:
+    return _sha256_hex(
+        _canonical(
+            {
+                "kind": kind,
+                "title": title,
+                "url": url,
+                "detail": detail or {},
+            }
+        )
+    )
+
+
+def mail_content_hash(to_addr: str | None, subject: str | None, body: str | None) -> str:
+    return _sha256_hex(
+        _canonical(
+            {
+                "to_addr": to_addr,
+                "subject": subject,
+                "body": body,
+            }
+        )
+    )
+
+
+def content_content_hash(channel: str, caption: str, link: str | None) -> str:
+    return _sha256_hex(
+        _canonical(
+            {
+                "channel": channel,
+                "caption": caption,
+                "link": link,
+            }
+        )
+    )
+
+
+def approval_message(
+    item_type: str, deployment_id: str, item_id: int, content_hash: str
+) -> bytes:
+    """The exact 32-byte message the operator signs."""
+    raw = f"cayce-ops:{item_type}:{deployment_id}:{item_id}:{content_hash}"
+    return hashlib.sha256(raw.encode("utf-8")).digest()
+
+
+def verify_signature(
+    item_type: str,
+    deployment_id: str,
+    item_id: int,
+    content_hash: str,
+    signature_hex: str | None,
+    operator_pubkey_hex: str,
+) -> bool:
+    """Verify an operator approval signature. Foreign deployments or edited
+    content verify False because both are inside the signed message."""
+    if not signature_hex or not operator_pubkey_hex or not deployment_id:
+        return False
+    from cayceops.components.publisher.nostr import _schnorr_verify
+
+    try:
+        return _schnorr_verify(
+            bytes.fromhex(signature_hex),
+            approval_message(item_type, deployment_id, item_id, content_hash),
+            bytes.fromhex(operator_pubkey_hex),
+        )
+    except (ValueError, TypeError):
+        return False