fix: keep tool-use/result pairs intact during compaction

Preserve assistant tool_use / user tool_result adjacency when compaction trims message history.

Fixes #325.

Squashed original PR commits:

- Fix compaction breaking tool-use/result pairs

- Simplify compaction boundary fix

s20_comprehensive/code.py

@@ -1060,6 +1060,26 @@ def spawn_subagent(description: str) -> str:
 def estimate_size(messages: list) -> int:
     return len(json.dumps(messages, default=str))
 
+def block_type(block):
+    return getattr(block, "type", None) if not isinstance(block, dict) else block.get("type")
+
+def has_tool_use(message: dict) -> bool:
+    if message.get("role") != "assistant":
+        return False
+    content = message.get("content")
+    if not isinstance(content, list):
+        return False
+    return any(block_type(block) == "tool_use" for block in content)
+
+def is_tool_result_message(message: dict) -> bool:
+    if message.get("role") != "user":
+        return False
+    content = message.get("content")
+    if not isinstance(content, list):
+        return False
+    return any(isinstance(block, dict) and block.get("type") == "tool_result"
+               for block in content)
+
 
 def collect_tool_results(messages: list):
     found = []
@@ -1111,11 +1131,18 @@ def tool_result_budget(messages: list, max_bytes: int = 200_000) -> list:
 def snip_compact(messages: list, max_messages: int = 50) -> list:
     if len(messages) <= max_messages:
         return messages
-    keep_head, keep_tail = 3, max_messages - 3
-    snipped = len(messages) - keep_head - keep_tail
-    return (messages[:keep_head]
+    head_end, tail_start = 3, len(messages) - (max_messages - 3)
+    if head_end > 0 and has_tool_use(messages[head_end - 1]):
+        while head_end < len(messages) and is_tool_result_message(messages[head_end]):
+            head_end += 1
+    if tail_start > 0 and tail_start < len(messages) and is_tool_result_message(messages[tail_start]) and has_tool_use(messages[tail_start - 1]):
+        tail_start -= 1
+    if head_end >= tail_start:
+        return messages
+    snipped = tail_start - head_end
+    return (messages[:head_end]
             + [{"role": "user", "content": f"[snipped {snipped} messages]"}]
-            + messages[-keep_tail:])
+            + messages[tail_start:])
 
 
 def micro_compact(messages: list) -> list:
@@ -1163,8 +1190,11 @@ def reactive_compact(messages: list) -> list:
         summary = summarize_history(messages)
     except Exception:
         summary = "Earlier conversation was trimmed after a prompt-too-long error."
+    tail_start = max(0, len(messages) - 5)
+    if tail_start > 0 and tail_start < len(messages) and is_tool_result_message(messages[tail_start]) and has_tool_use(messages[tail_start - 1]):
+        tail_start -= 1
     return [{"role": "user", "content": f"[Reactive compact]\n\n{summary}"},
-            *messages[-5:]]
+            *messages[tail_start:]]
 
 
 # ── Error Recovery ──