feat(config): wire max_messages into session history replay

The max_messages config field in AgentDefaults was accepted by the
schema but never threaded through to the actual get_history() calls
in the agent loop.  Both call sites in _process_message hardcoded the
default, so sessions with slow or local models accumulated unbounded
history that inflated prompt tokens and caused LLM timeouts.

Changes:
- Add max_messages field to AgentDefaults (default 0 = use built-in
  constant, any positive value caps history replay)
- Store the value on AgentLoop and pass it to get_history() when
  non-zero
- Wire the config through all three AgentLoop construction sites in
  commands.py (gateway, API server, CLI chat)
- 14 focused tests covering schema validation, init storage, history
  slicing, boundary alignment, integration wiring, and the
  zero/default path

nanobot/cli/commands.py

@@ -538,6 +538,7 @@ def serve(
         disabled_skills=runtime_config.agents.defaults.disabled_skills,
         session_ttl_minutes=runtime_config.agents.defaults.session_ttl_minutes,
         consolidation_ratio=runtime_config.agents.defaults.consolidation_ratio,
+        max_messages=runtime_config.agents.defaults.max_messages,
         tools_config=runtime_config.tools,
     )
 
@@ -651,6 +652,7 @@ def _run_gateway(
         disabled_skills=config.agents.defaults.disabled_skills,
         session_ttl_minutes=config.agents.defaults.session_ttl_minutes,
         consolidation_ratio=config.agents.defaults.consolidation_ratio,
+        max_messages=config.agents.defaults.max_messages,
         tools_config=config.tools,
         provider_snapshot_loader=load_provider_snapshot,
         provider_signature=provider_snapshot.signature,
@@ -1043,6 +1045,7 @@ def agent(
         disabled_skills=config.agents.defaults.disabled_skills,
         session_ttl_minutes=config.agents.defaults.session_ttl_minutes,
         consolidation_ratio=config.agents.defaults.consolidation_ratio,
+        max_messages=config.agents.defaults.max_messages,
         tools_config=config.tools,
     )
     restart_notice = consume_restart_notice_from_env()