feat: add agent-scoped agent_prelude config

[sigoden]

The agent_prelude config field in config.yaml provides global control over the agent prelude. However, each agent has its own config.yaml file, which can also contain an agent_prelude field for agent-specific control.

# <aichat-config-dir>/config.yaml
agent_prelude: 'temp'

# <aichat-config-dir>/agents/demo/config.yaml
agent_prelude: ''

When starting in demo agent, it will not use the temp session automaticlly.