feat(cli): display model reasoning content during streaming

Add show_reasoning config (default: False) to display model
thinking/reasoning content in the TUI during streaming.  Reasoning
is emitted via a new emit_reasoning hook on AgentHook, gated by the
channels config.  Display uses ✻ prefix with dim italic styling.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/agent/test_hook_composite.py

@@ -13,6 +13,17 @@ def _ctx() -> AgentHookContext:
     return AgentHookContext(iteration=0, messages=[])
 
 
+# ---------------------------------------------------------------------------
+# Base AgentHook emit_reasoning: no-op
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_base_hook_emit_reasoning_is_noop():
+    hook = AgentHook()
+    await hook.emit_reasoning("should not raise")
+
+
 # ---------------------------------------------------------------------------
 # Fan-out: every hook is called in order
 # ---------------------------------------------------------------------------
@@ -45,6 +56,9 @@ async def test_composite_fans_out_all_async_methods():
         async def before_iteration(self, context: AgentHookContext) -> None:
             events.append("before_iteration")
 
+        async def emit_reasoning(self, reasoning_content: str | None) -> None:
+            events.append(f"emit_reasoning:{reasoning_content}")
+
         async def on_stream(self, context: AgentHookContext, delta: str) -> None:
             events.append(f"on_stream:{delta}")
 
@@ -61,6 +75,7 @@ async def test_composite_fans_out_all_async_methods():
     ctx = _ctx()
 
     await hook.before_iteration(ctx)
+    await hook.emit_reasoning("thinking...")
     await hook.on_stream(ctx, "hi")
     await hook.on_stream_end(ctx, resuming=True)
     await hook.before_execute_tools(ctx)
@@ -68,6 +83,7 @@ async def test_composite_fans_out_all_async_methods():
 
     assert events == [
         "before_iteration", "before_iteration",
+        "emit_reasoning:thinking...", "emit_reasoning:thinking...",
         "on_stream:hi", "on_stream:hi",
         "on_stream_end:True", "on_stream_end:True",
         "before_execute_tools", "before_execute_tools",
@@ -120,6 +136,8 @@ async def test_composite_error_isolation_all_async():
     calls: list[str] = []
 
     class Bad(AgentHook):
+        async def emit_reasoning(self, reasoning_content):
+            raise RuntimeError("err")
         async def on_stream_end(self, context, *, resuming):
             raise RuntimeError("err")
         async def before_execute_tools(self, context):
@@ -128,6 +146,8 @@ async def test_composite_error_isolation_all_async():
             raise RuntimeError("err")
 
     class Good(AgentHook):
+        async def emit_reasoning(self, reasoning_content):
+            calls.append("emit_reasoning")
         async def on_stream_end(self, context, *, resuming):
             calls.append("on_stream_end")
         async def before_execute_tools(self, context):
@@ -137,10 +157,11 @@ async def test_composite_error_isolation_all_async():
 
     hook = CompositeHook([Bad(), Good()])
     ctx = _ctx()
+    await hook.emit_reasoning("test")
     await hook.on_stream_end(ctx, resuming=False)
     await hook.before_execute_tools(ctx)
     await hook.after_iteration(ctx)
-    assert calls == ["on_stream_end", "before_execute_tools", "after_iteration"]
+    assert calls == ["emit_reasoning", "on_stream_end", "before_execute_tools", "after_iteration"]
 
 
 # ---------------------------------------------------------------------------