fix(error-classifier): route 5xx context-overflow into compression

Local inference servers (llama.cpp/llama-server, vLLM/Ollama behind a
Cloudflare/Tailscale hop) report context overflow with HTTP 500/502/503/529
instead of 400/413. _classify_by_status returned server_error/overloaded and
retried blindly, then dropped the turn with no compaction. Route explicit
_CONTEXT_OVERFLOW_PATTERNS matches on those 5xx codes to context_overflow
(should_compress=True); plain 500 stays server_error, plain 503 overloaded.

agent/error_classifier.py

@@ -964,9 +964,31 @@ def _classify_by_status(
                 retryable=False,
                 should_fallback=True,
             )
+        # Some local inference servers (notably llama.cpp / llama-server)
+        # report context overflow with an HTTP 500 instead of the standard
+        # 400/413. The request-validation guard above already ran, so any
+        # remaining explicit context-overflow signal routes into the
+        # compression-and-retry path (mirroring _classify_400) instead of
+        # blind server_error retries that exhaust and drop the turn.
+        if any(p in error_msg for p in _CONTEXT_OVERFLOW_PATTERNS):
+            return result_fn(
+                FailoverReason.context_overflow,
+                retryable=True,
+                should_compress=True,
+            )
         return result_fn(FailoverReason.server_error, retryable=True)
 
     if status_code in {503, 529}:
+        # Same overflow-as-5xx variant (server busy / model-load OOM, or a
+        # Cloudflare/Tailscale hop relabeling the status). Route explicit
+        # overflow bodies into compression; otherwise treat as transient
+        # overload and retry.
+        if any(p in error_msg for p in _CONTEXT_OVERFLOW_PATTERNS):
+            return result_fn(
+                FailoverReason.context_overflow,
+                retryable=True,
+                should_compress=True,
+            )
         return result_fn(FailoverReason.overloaded, retryable=True)
 
     # Other 4xx — non-retryable