feat(tui): fix lint, add robot/benchmark/changelog for pruning

- Fix ruff format violations in app.py, conversation.py, and
  tui_conversation_pruning_steps.py (3 files reformatted)
- Fix ruff E501 line-too-long in helper_tui_conversation_pruning.py
- Expand ConversationSettings docstring to explain hysteresis design
- Add _note_active invariant comment in ConversationStream.__init__
- Add full docstring to load_conversation_settings (config path, keys,
  fallback behaviour)
- Narrow bare except Exception to (OSError, json.JSONDecodeError) and
  (ValueError, TypeError) in load_conversation_settings
- Add Robot Framework integration tests (tui_conversation_pruning.robot
  + helper) covering prune-trigger, note-inserted, protected-blocks,
  clear-resets-state, settings-defaults
- Add ASV benchmarks (conversation_stream_bench.py) covering add_block
  baseline, heavy-load pruning, render after prune, clear+repopulate
- Add CHANGELOG.md entry for feat(tui) conversation content pruning

ISSUES CLOSED: #6350

benchmarks/conversation_stream_bench.py

@@ -0,0 +1,99 @@
+"""ASV benchmarks for ConversationStream pruning performance.
+
+Measures the performance of:
+- ConversationStream.add_block() with no pruning (baseline)
+- ConversationStream.add_block() under heavy load (pruning fires on every block)
+- ConversationStream.render() on a pruned stream
+- ConversationStream.clear() followed by repopulation
+"""
+
+from __future__ import annotations
+
+import sys
+from pathlib import Path
+
+try:
+    from cleveragents.tui.conversation import ConversationSettings, ConversationStream
+except ModuleNotFoundError:
+    sys.path.insert(0, str(Path(__file__).resolve().parents[1] / "src"))
+    from cleveragents.tui.conversation import ConversationSettings, ConversationStream
+
+
+def _make_stream(
+    prune_low_mark: int = 1_500,
+    prune_excess: int = 1_000,
+    preserve_recent_lines: int = 500,
+) -> ConversationStream:
+    settings = ConversationSettings(
+        prune_low_mark=prune_low_mark,
+        prune_excess=prune_excess,
+        preserve_recent_lines=preserve_recent_lines,
+    )
+    return ConversationStream(settings=settings)
+
+
+_BLOCK_10_LINES = "\n".join(f"bench line {i}" for i in range(10))
+_BLOCK_50_LINES = "\n".join(f"bench line {i}" for i in range(50))
+
+
+class ConversationStreamAddBlockSuite:
+    """Benchmark ConversationStream.add_block under varying load."""
+
+    def setup(self):
+        # Pre-built stream near (but not at) the default trigger threshold.
+        self.stream_near_threshold = _make_stream()
+        for i in range(240):
+            self.stream_near_threshold.add_block(_BLOCK_10_LINES, block_type="message")
+        # (240 * 10 = 2400 lines, trigger = 2500 — still below threshold)
+
+    def time_add_block_no_prune(self):
+        """Baseline: add a single block to an empty stream (no pruning)."""
+        stream = _make_stream()
+        stream.add_block(_BLOCK_10_LINES, block_type="message")
+
+    def time_add_block_triggers_prune(self):
+        """Add one block past the threshold so pruning fires once."""
+        stream = _make_stream()
+        for i in range(241):
+            stream.add_block(_BLOCK_10_LINES, block_type="message")
+
+    def time_add_block_heavy_load(self):
+        """Add 500 blocks to a tight-threshold stream (pruning fires repeatedly)."""
+        stream = _make_stream(prune_low_mark=100, prune_excess=50)
+        for _ in range(500):
+            stream.add_block(_BLOCK_10_LINES, block_type="message")
+
+    def time_add_block_large_blocks(self):
+        """Add 50-line blocks; fewer blocks needed to trigger pruning."""
+        stream = _make_stream(prune_low_mark=100, prune_excess=50)
+        for _ in range(100):
+            stream.add_block(_BLOCK_50_LINES, block_type="message")
+
+
+class ConversationStreamRenderSuite:
+    """Benchmark ConversationStream.render() after pruning."""
+
+    def setup(self):
+        self.stream = _make_stream(prune_low_mark=100, prune_excess=50)
+        for i in range(200):
+            self.stream.add_block(_BLOCK_10_LINES, block_type="message")
+
+    def time_render_pruned_stream(self):
+        """Benchmark render() on a stream that has been pruned."""
+        self.stream.render()
+
+
+class ConversationStreamClearSuite:
+    """Benchmark ConversationStream.clear() and repopulation."""
+
+    def setup(self):
+        self.block = _BLOCK_10_LINES
+
+    def time_clear_and_repopulate(self):
+        """Benchmark clear() followed by adding 50 blocks."""
+        stream = _make_stream()
+        for _ in range(50):
+            stream.add_block(self.block, block_type="message")
+        stream.clear()
+        for _ in range(50):
+            stream.add_block(self.block, block_type="message")