fix(compression): replace dead summary_target_tokens with ratio-based scaling (#2554)

The summary_target_tokens parameter was accepted in the constructor,
stored on the instance, and never used — the summary budget was always
computed from hardcoded module constants (_SUMMARY_RATIO=0.20,
_MAX_SUMMARY_TOKENS=8000). This caused two compounding problems:

1. The config value was silently ignored, giving users no control
   over post-compression size.
2. Fixed budgets (20K tail, 8K summary cap) didn't scale with
   context window size. Switching from a 1M-context model to a
   200K model would trigger compression that nuked 350K tokens
   of conversation history down to ~30K.

Changes:
- Replace summary_target_tokens with summary_target_ratio (default 0.40)
  which sets the post-compression target as a fraction of context_length.
  Tail token budget and summary cap now scale proportionally:
    MiniMax 200K → ~80K post-compression
    GPT-5   1M  → ~400K post-compression
- Change threshold_percent default: 0.50 → 0.80 (don't fire until
  80% of context is consumed)
- Change protect_last_n default: 4 → 20 (preserve ~10 full turns)
- Summary token cap scales to 5% of context (was fixed 8K), capped
  at 32K ceiling
- Read target_ratio and protect_last_n from config.yaml compression
  section (both are now configurable)
- Remove hardcoded summary_target_tokens=500 from run_agent.py
- Add 5 new tests for ratio scaling, clamping, and new defaults

run_agent.py

@@ -1009,9 +1009,11 @@ class AIAgent:
         _compression_cfg = _agent_cfg.get("compression", {})
         if not isinstance(_compression_cfg, dict):
             _compression_cfg = {}
-        compression_threshold = float(_compression_cfg.get("threshold", 0.50))
+        compression_threshold = float(_compression_cfg.get("threshold", 0.80))
         compression_enabled = str(_compression_cfg.get("enabled", True)).lower() in ("true", "1", "yes")
         compression_summary_model = _compression_cfg.get("summary_model") or None
+        compression_target_ratio = float(_compression_cfg.get("target_ratio", 0.40))
+        compression_protect_last = int(_compression_cfg.get("protect_last_n", 20))
 
         # Read explicit context_length override from model config
         _model_cfg = _agent_cfg.get("model", {})
@@ -1050,8 +1052,8 @@ class AIAgent:
             model=self.model,
             threshold_percent=compression_threshold,
             protect_first_n=3,
-            protect_last_n=4,
-            summary_target_tokens=500,
+            protect_last_n=compression_protect_last,
+            summary_target_ratio=compression_target_ratio,
             summary_model_override=compression_summary_model,
             quiet_mode=self.quiet_mode,
             base_url=self.base_url,