refactor: introduce AgentLoop.from_config() to centralize loop assembly

Extract duplicated bus/provider/loop initialization from CLI commands
(serve, _run_gateway, agent) and Nanobot facade into a single
AgentLoop.from_config() classmethod.

- Remove _make_provider() from cli/commands.py and nanobot.py
- Remove inline provider creation in all three CLI entry points
- AgentLoop.from_config() creates MessageBus, calls make_provider(),
  and assembles AgentLoop with all standard config-derived parameters
- Supports **extra overrides for callers that need custom args
  (e.g. cron_service, session_manager, provider_snapshot_loader)
- Update tests to mock make_provider at nanobot.providers.factory
  and add from_config classmethod to _FakeAgentLoop fixtures

This is PR 1/4 of the model-preset feature decomposition.

nanobot/nanobot.py

@@ -8,7 +8,6 @@ from typing import Any
 
 from nanobot.agent.hook import AgentHook, SDKCaptureHook
 from nanobot.agent.loop import AgentLoop
-from nanobot.bus.queue import MessageBus
 
 
 @dataclass(slots=True)
@@ -62,31 +61,8 @@ class Nanobot:
                 Path(workspace).expanduser().resolve()
             )
 
-        provider = _make_provider(config)
-        bus = MessageBus()
-        defaults = config.agents.defaults
-
-        loop = AgentLoop(
-            bus=bus,
-            provider=provider,
-            workspace=config.workspace_path,
-            model=defaults.model,
-            max_iterations=defaults.max_tool_iterations,
-            context_window_tokens=defaults.context_window_tokens,
-            context_block_limit=defaults.context_block_limit,
-            max_tool_result_chars=defaults.max_tool_result_chars,
-            provider_retry_mode=defaults.provider_retry_mode,
-            tool_hint_max_length=defaults.tool_hint_max_length,
-            web_config=config.tools.web,
-            exec_config=config.tools.exec,
-            restrict_to_workspace=config.tools.restrict_to_workspace,
-            mcp_servers=config.tools.mcp_servers,
-            timezone=defaults.timezone,
-            unified_session=defaults.unified_session,
-            disabled_skills=defaults.disabled_skills,
-            session_ttl_minutes=defaults.session_ttl_minutes,
-            consolidation_ratio=defaults.consolidation_ratio,
-            tools_config=config.tools,
+        loop = AgentLoop.from_config(
+            config,
             image_generation_provider_configs={
                 "openrouter": config.providers.openrouter,
                 "aihubmix": config.providers.aihubmix,
@@ -128,8 +104,3 @@ class Nanobot:
         )
 
 
-def _make_provider(config: Any) -> Any:
-    """Create the LLM provider from config (extracted from CLI)."""
-    from nanobot.providers.factory import make_provider
-
-    return make_provider(config)