feat(runner): support structured fallback models

Bind fallback model chains to the active model configuration so defaults and presets do not inherit or merge fallback behavior implicitly. Require explicit fallback providers while preserving per-fallback generation overrides and context-window safety.

Co-authored-by: Cursor <cursoragent@cursor.com>

nanobot/config/schema.py

@@ -74,6 +74,17 @@ class DreamConfig(Base):
         return f"every {hours}h"
 
 
+class ModelFallbackConfig(Base):
+    """A fallback model tied to one active model configuration."""
+
+    model: str
+    provider: str
+    max_tokens: int | None = None
+    context_window_tokens: int | None = None
+    temperature: float | None = None
+    reasoning_effort: str | None = None
+
+
 class ModelPresetConfig(Base):
     """A named set of model + generation parameters for quick switching."""
 
@@ -83,7 +94,7 @@ class ModelPresetConfig(Base):
     context_window_tokens: int = 65_536
     temperature: float = 0.1
     reasoning_effort: str | None = None
-    fallback_models: list[str] = Field(default_factory=list)
+    fallback_models: list[ModelFallbackConfig] = Field(default_factory=list)
 
     def to_generation_settings(self) -> Any:
         from nanobot.providers.base import GenerationSettings
@@ -107,6 +118,7 @@ class AgentDefaults(Base):
     context_window_tokens: int = 65_536
     context_block_limit: int | None = None
     temperature: float = 0.1
+    fallback_models: list[ModelFallbackConfig] = Field(default_factory=list)
     max_tool_iterations: int = 200
     max_concurrent_subagents: int = Field(default=1, ge=1)
     max_tool_result_chars: int = 16_000
@@ -297,6 +309,7 @@ class Config(BaseSettings):
             model=d.model, provider=d.provider, max_tokens=d.max_tokens,
             context_window_tokens=d.context_window_tokens,
             temperature=d.temperature, reasoning_effort=d.reasoning_effort,
+            fallback_models=d.fallback_models,
         )
 
     def resolve_preset(self, name: str | None = None) -> ModelPresetConfig: