fix: reuse token truncation helper

Maintainer edit: make token truncation include the suffix within the budget and route the consolidator through the shared helper so recent-history and archive truncation keep the same semantics.

tests/agent/test_context_prompt_cache.py

@@ -237,8 +237,7 @@ def test_recent_history_truncated_at_max_tokens(tmp_path) -> None:
     assert len(history_section) == 2
 
     enc = tiktoken.get_encoding("cl100k_base")
-    # Small margin for the truncation suffix appended after the token slice.
-    assert len(enc.encode(history_section[1])) <= builder._MAX_HISTORY_TOKENS + 50
+    assert len(enc.encode(history_section[1])) <= builder._MAX_HISTORY_TOKENS
 
 
 def test_no_recent_history_when_dream_has_processed_all(tmp_path) -> None: