fix(memory): do not fall back to raw entry when strip_think empties it

`append_history` previously used `strip_think(entry) or entry.rstrip()`
as a safety net, so if the entire entry was a template-token leak (e.g.
`<think>reasoning</think>` or `<channel|>` alone), the raw leaked text
was still persisted to history — later re-introducing the very content
`strip_think` was meant to scrub, via consolidation / replay.

Persist the cleaned content directly. When cleanup empties a non-empty
entry, log at debug and store an empty-content record (cursor continuity
preserved). Adds 3 regression tests in test_memory_store.py covering:

  - Well-formed thinking blocks are stripped before persistence.
  - Pure-leak entries persist as empty, not as raw text.
  - Malformed prefix leaks (`<channel|>`) also persist as empty.

nanobot/agent/memory.py

@@ -221,10 +221,26 @@ class MemoryStore:
     # -- history.jsonl — append-only, JSONL format ---------------------------
 
     def append_history(self, entry: str) -> int:
-        """Append *entry* to history.jsonl and return its auto-incrementing cursor."""
+        """Append *entry* to history.jsonl and return its auto-incrementing cursor.
+
+        Entries are passed through `strip_think` to drop template-level leaks
+        (e.g. unclosed `<think` prefixes, `<channel|>` markers) before being
+        persisted. If the cleaned content is empty but the raw entry wasn't,
+        the record is persisted with an empty string rather than falling back
+        to the raw leak — otherwise `strip_think`'s guarantees would be
+        undone by history replay / consolidation downstream.
+        """
         cursor = self._next_cursor()
         ts = datetime.now().strftime("%Y-%m-%d %H:%M")
-        record = {"cursor": cursor, "timestamp": ts, "content": strip_think(entry.rstrip()) or entry.rstrip()}
+        raw = entry.rstrip()
+        content = strip_think(raw)
+        if raw and not content:
+            logger.debug(
+                "history entry {} stripped to empty (likely template leak); "
+                "persisting empty content to avoid re-polluting context",
+                cursor,
+            )
+        record = {"cursor": cursor, "timestamp": ts, "content": content}
         with open(self.history_file, "a", encoding="utf-8") as f:
             f.write(json.dumps(record, ensure_ascii=False) + "\n")
         self._cursor_file.write_text(str(cursor), encoding="utf-8")