feat(gateway): make hygiene hard message limit configurable (#17000)

The gateway session-hygiene pre-compression safety valve had a hardcoded
400-message threshold. On long-lived sessions with short turns this was
either too high (users with aggressive compression preferences) or too
low (users with very large context models who want to keep more history
in-flight).

Add compression.hygiene_hard_message_limit (default 400) so it can be
tuned without forking the gateway.

Reported by @OP (Apr 26 feedback bundle).

## Changes
- hermes_cli/config.py: new DEFAULT_CONFIG key with 400 default
- gateway/run.py: read compression.hygiene_hard_message_limit at
  hygiene-time, fall back to 400 if missing/invalid
- tests/gateway/test_session_hygiene.py: two tests — override fires at
  the configured limit, default does not fire below 400

Co-authored-by: teknium1 <teknium@users.noreply.github.com>

hermes_cli/config.py

@@ -582,7 +582,7 @@ DEFAULT_CONFIG = {
         "threshold": 0.50,            # compress when context usage exceeds this ratio
         "target_ratio": 0.20,         # fraction of threshold to preserve as recent tail
         "protect_last_n": 20,         # minimum recent messages to keep uncompressed
-
+        "hygiene_hard_message_limit": 400,  # gateway session-hygiene force-compress threshold by message count
     },
 
     # Anthropic prompt caching (Claude via OpenRouter or native Anthropic API).