Define AgentEvent type and JSON serialization

Create Omni/Agent/Event.hs with:

1. AgentEvent sum type for all event kinds 2. ToJSON/FromJSON instances 3. Helper constructors 4. Session ID generation (UUID or timestamp-based)

data AgentEvent = AgentEvent
  { eventType :: EventType
  , eventTimestamp :: UTCTime
  , eventContent :: Value  -- type-specific payload
  }

data EventType
  = Assistant    -- LLM text response
  | ToolCall     -- tool invocation with args
  | ToolResult   -- tool output
  | UserMessage  -- for future interactive chat
  | Cost         -- tokens and cost
  | Error        -- failures
  | Complete     -- session ended