fix: prevent agent from stopping mid-task — compression floor, budget overhaul, activity tracking

Three root causes of the 'agent stops mid-task' gateway bug:

1. Compression threshold floor (64K tokens minimum)
   - The 50% threshold on a 100K-context model fired at 50K tokens,
     causing premature compression that made models lose track of
     multi-step plans.  Now threshold_tokens = max(50% * context, 64K).
   - Models with <64K context are rejected at startup with a clear error.

2. Budget warning removal — grace call instead
   - Removed the 70%/90% iteration budget warnings entirely.  These
     injected '[BUDGET WARNING: Provide your final response NOW]' into
     tool results, causing models to abandon complex tasks prematurely.
   - Now: no warnings during normal execution.  When the budget is
     actually exhausted (90/90), inject a user message asking the model
     to summarise, allow one grace API call, and only then fall back
     to _handle_max_iterations.

3. Activity touches during long terminal execution
   - _wait_for_process polls every 0.2s but never reported activity.
     The gateway's inactivity timeout (default 1800s) would fire during
     long-running commands that appeared 'idle.'
   - Now: thread-local activity callback fires every 10s during the
     poll loop, keeping the gateway's activity tracker alive.
   - Agent wires _touch_activity into the callback before each tool call.

Also: docs update noting 64K minimum context requirement.

Closes #7915 (root cause was agent-loop termination, not Weixin delivery limits).

tests/agent/test_context_compressor.py

@@ -576,11 +576,19 @@ class TestSummaryTargetRatio:
         assert c.summary_target_ratio == 0.80
 
     def test_default_threshold_is_50_percent(self):
-        """Default compression threshold should be 50%."""
+        """Default compression threshold should be 50%, with a 64K floor."""
         with patch("agent.context_compressor.get_model_context_length", return_value=100_000):
             c = ContextCompressor(model="test", quiet_mode=True)
         assert c.threshold_percent == 0.50
-        assert c.threshold_tokens == 50_000
+        # 50% of 100K = 50K, but the floor is 64K
+        assert c.threshold_tokens == 64_000
+
+    def test_threshold_floor_does_not_apply_above_128k(self):
+        """On large-context models the 50% percentage is used directly."""
+        with patch("agent.context_compressor.get_model_context_length", return_value=200_000):
+            c = ContextCompressor(model="test", quiet_mode=True)
+        # 50% of 200K = 100K, which is above the 64K floor
+        assert c.threshold_tokens == 100_000
 
     def test_default_protect_last_n_is_20(self):
         """Default protect_last_n should be 20."""