fix(loop): strip input_audio and video_url before session persistence

Code review found that _sanitize_persisted_blocks only stripped
image_url blocks, causing base64-encoded audio/video payloads to
bloat session history files.

- Extend _sanitize_persisted_blocks to replace input_audio and
  video_url blocks with text placeholders using LLMProvider._media_placeholder.
- Add tests for audio/video stripping with and without _meta.path.

All 3267 tests pass.

nanobot/agent/loop.py

@@ -1422,6 +1422,10 @@ class AgentLoop:
                 filtered.append({"type": "text", "text": image_placeholder_text(path)})
                 continue
 
+            if block.get("type") in ("input_audio", "video_url"):
+                filtered.append(LLMProvider._media_placeholder(block["type"], block))
+                continue
+
             if block.get("type") == "text" and isinstance(block.get("text"), str):
                 text = block["text"]
                 if should_truncate_text and len(text) > self.max_tool_result_chars:

tests/test_truncate_text_shadowing.py

@@ -29,3 +29,47 @@ def test_sanitize_persisted_blocks_truncate_text_shadowing_regression() -> None:
     assert isinstance(out[0]["text"], str)
     assert out[0]["text"] != content[0]["text"]
 
+
+def test_sanitize_persisted_blocks_strips_audio_and_video() -> None:
+    """Audio and video blocks with base64 payloads must be replaced with placeholders."""
+    from nanobot.agent.loop import AgentLoop
+
+    dummy = SimpleNamespace(max_tool_result_chars=1000)
+    content = [
+        {"type": "text", "text": "analyze this"},
+        {
+            "type": "input_audio",
+            "input_audio": {"data": "aGVsbG8=", "format": "wav"},
+            "_meta": {"path": "/tmp/voice.wav"},
+        },
+        {
+            "type": "video_url",
+            "video_url": {"url": "data:video/mp4;base64,aGVsbG8="},
+            "_meta": {"path": "/tmp/clip.mp4"},
+        },
+    ]
+
+    out = AgentLoop._sanitize_persisted_blocks(dummy, content)
+
+    assert len(out) == 3
+    assert out[0] == content[0]
+    assert out[1] == {"type": "text", "text": "[audio: /tmp/voice.wav]"}
+    assert out[2] == {"type": "text", "text": "[video: /tmp/clip.mp4]"}
+
+
+def test_sanitize_persisted_blocks_strips_audio_video_without_meta() -> None:
+    """When _meta is absent, fallback placeholders use bare label."""
+    from nanobot.agent.loop import AgentLoop
+
+    dummy = SimpleNamespace(max_tool_result_chars=1000)
+    content = [
+        {"type": "input_audio", "input_audio": {"data": "aGVsbG8=", "format": "wav"}},
+        {"type": "video_url", "video_url": {"url": "data:video/mp4;base64,aGVsbG8="}},
+    ]
+
+    out = AgentLoop._sanitize_persisted_blocks(dummy, content)
+
+    assert len(out) == 2
+    assert out[0] == {"type": "text", "text": "[audio]"}
+    assert out[1] == {"type": "text", "text": "[video]"}
+