Create Legacy Wrapper for Existing Engine

Create a wrapper that exposes current Engine.runAgentWithProvider as an Op.

Context

We want to migrate incrementally. This wrapper lets us use the new Op interface while still using the battle-tested Engine implementation internally.

Deliverables

Create Omni/Agent/Op/Legacy.hs containing:

1. Legacy Agent Op

-- | Run the legacy engine as an Op
-- This wraps Engine.runAgentWithProvider so existing behavior is preserved
-- while we gradually migrate to native Op implementation
legacyAgent :: AgentConfig -> Text -> Op AgentState AgentResult
legacyAgent config prompt = do
  -- Emit start event
  emit (EventCustom "legacy_agent_start" (toJSON config))
  
  -- Create a provider from config (or take as parameter)
  -- Run the existing engine
  result <- liftIO (Engine.runAgentWithProvider engineConfig provider config prompt)
  
  case result of
    Left err -> do
      emit (EventCustom "legacy_agent_error" (toJSON err))
      error err  -- or return Either
    Right agentResult -> do
      emit (EventCustom "legacy_agent_complete" (toJSON agentResult))
      pure agentResult

2. State Adapter

-- | Standard agent state that matches Engine's implicit state
data AgentState = AgentState
  { asHistory :: [Message]
  , asTotalCost :: Double
  , asTotalTokens :: Int
  , asIteration :: Int
  }
  deriving (Show, Eq, Generic)

instance ToJSON AgentState
instance FromJSON AgentState

initialAgentState :: AgentState

3. Config Adapter

-- | Convert between Op-style config and Engine-style config
toEngineConfig :: OpConfig -> Engine.EngineConfig
toAgentConfig :: OpConfig -> Engine.AgentConfig

4. LiftIO for Op

We need a way to run IO in Op:

-- Add to Op.hs if not there:
data OpF s next where
  ...
  LiftIO :: IO a -> (a -> next) -> OpF s next

liftIO :: IO a -> Op s a
liftIO io = liftF (LiftIO io id)

Or use a different approach (ReaderT IO, etc.)

Notes

• This is a bridge for incremental migration
• Should produce same results as calling Engine directly
• Events emitted should be compatible with new Trace format
• Once native Op implementation is complete, this can be deprecated

Testing

• legacyAgent produces same result as Engine.runAgentWithProvider
• Events are captured in trace
• Can run legacyAgent through Sequential interpreter

Files to Read First

• Omni/Agent/Op.hs
• Omni/Agent/Engine.hs
• Omni/Agent/Interpreter/Sequential.hs