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).

agent/context_compressor.py

@@ -20,6 +20,7 @@ from typing import Any, Dict, List, Optional
 from agent.auxiliary_client import call_llm
 from agent.context_engine import ContextEngine
 from agent.model_metadata import (
+    MINIMUM_CONTEXT_LENGTH,
     get_model_context_length,
     estimate_messages_tokens_rough,
 )
@@ -87,7 +88,10 @@ class ContextCompressor(ContextEngine):
         self.api_key = api_key
         self.provider = provider
         self.context_length = context_length
-        self.threshold_tokens = int(context_length * self.threshold_percent)
+        self.threshold_tokens = max(
+            int(context_length * self.threshold_percent),
+            MINIMUM_CONTEXT_LENGTH,
+        )
 
     def __init__(
         self,
@@ -118,7 +122,14 @@ class ContextCompressor(ContextEngine):
             config_context_length=config_context_length,
             provider=provider,
         )
-        self.threshold_tokens = int(self.context_length * threshold_percent)
+        # Floor: never compress below MINIMUM_CONTEXT_LENGTH tokens even if
+        # the percentage would suggest a lower value.  This prevents premature
+        # compression on large-context models at 50% while keeping the % sane
+        # for models right at the minimum.
+        self.threshold_tokens = max(
+            int(self.context_length * threshold_percent),
+            MINIMUM_CONTEXT_LENGTH,
+        )
         self.compression_count = 0
 
         # Derive token budgets: ratio is relative to the threshold, not total context