fix(storage): sanitize NUL bytes on _source + _reminders columns

Apply sanitize_text() to the new _source and _reminders columns in
both save_message and save_messages_bulk on SQLite + PostgreSQL,
mirroring the existing pattern used for content and provider_data.

Producers (sanitize_payload on the watch dispatch path,
format_nudge constants on the standard nudge path) already strip
NUL bytes today so nothing in production reaches this clamp — but
the storage layer is opaque to those invariants, and PostgreSQL
TEXT columns reject NUL outright.  Without this clamp, a future
producer that forgets sanitize_payload (or hand-builds the column
string) hard-fails the chat-loop persist path on PostgreSQL.

Cost is negligible — sanitize_text early-exits on the common
no-NUL case via 'if value and "\x00" in value'.

Surfaced by Copilot's PR #486 review.

(cherry picked from commit fc8bd6ca33)

tests/test_storage_source_reminders.py

@@ -105,6 +105,35 @@ class TestRemindersRoundtrip:
         assert len(tool_msgs) == 1
         assert tool_msgs[0].get("_reminders") == payload
 
+    def test_nul_bytes_stripped_from_source_and_reminders(self, backend):
+        """NUL bytes must be stripped at the storage layer.
+
+        Producers (``sanitize_payload`` on the watch dispatch path,
+        constants for non-watch nudges) already strip NUL today so
+        nothing in production reaches this clamp — but the layer is
+        the tripwire if a future producer forgets, mirroring how
+        ``content`` and ``provider_data`` are sanitized.  PostgreSQL
+        TEXT columns reject NUL outright, so the sanitization is also
+        a hard correctness invariant on that backend.
+
+        ``json.dumps`` already escapes NUL inside string values to
+        ``\\u0000`` so a real NUL byte can't enter ``_reminders`` via
+        the normal encode path — the test feeds a raw NUL directly to
+        cover the bypass case (a future producer that hand-builds the
+        column string).
+        """
+        backend.register_workstream("s1")
+        backend.save_message(
+            "s1",
+            "user",
+            "",
+            source="system_nudge\x00",
+            reminders='[{"type":"watch_triggered","text":"ok\x00bad"}]',
+        )
+        msgs = backend.load_messages("s1")
+        assert msgs[0].get("_source") == "system_nudge"
+        assert msgs[0].get("_reminders") == [{"type": "watch_triggered", "text": "okbad"}]
+
     def test_malformed_reminders_json_does_not_crash_load(self, backend):
         """A garbage string in the column must not abort the whole
         load — mirrors the ``provider_data`` JSON-decode-suppress

turnstone/core/storage/_postgresql.py

@@ -184,6 +184,8 @@ class PostgreSQLBackend:
         now = datetime.now(UTC).strftime("%Y-%m-%dT%H:%M:%S")
         content = sanitize_text(content)
         provider_data = sanitize_text(provider_data)
+        source = sanitize_text(source)
+        reminders = sanitize_text(reminders)
         with self._conn() as conn:
             result = conn.execute(
                 sa.insert(conversations)
@@ -227,8 +229,8 @@ class PostgreSQLBackend:
                     "tool_call_id": row.get("tool_call_id"),
                     "provider_data": sanitize_text(row.get("provider_data")),
                     "tool_calls": row.get("tool_calls"),
-                    "_source": row.get("source"),
-                    "_reminders": row.get("reminders"),
+                    "_source": sanitize_text(row.get("source")),
+                    "_reminders": sanitize_text(row.get("reminders")),
                 }
             )
         with self._conn() as conn:

turnstone/core/storage/_sqlite.py

@@ -224,6 +224,8 @@ class SQLiteBackend:
         now = datetime.now(UTC).strftime("%Y-%m-%dT%H:%M:%S")
         content = sanitize_text(content)
         provider_data = sanitize_text(provider_data)
+        source = sanitize_text(source)
+        reminders = sanitize_text(reminders)
         with self._conn() as conn:
             result = conn.execute(
                 sa.insert(conversations),
@@ -281,8 +283,8 @@ class SQLiteBackend:
                     "tool_call_id": row.get("tool_call_id"),
                     "provider_data": sanitize_text(row.get("provider_data")),
                     "tool_calls": row.get("tool_calls"),
-                    "_source": row.get("source"),
-                    "_reminders": row.get("reminders"),
+                    "_source": sanitize_text(row.get("source")),
+                    "_reminders": sanitize_text(row.get("reminders")),
                 }
             )
         with self._conn() as conn: