feat: allow overriding execute/code role

[sigoden]

There are two ways to override execution/code roles

1. use -r/--role

aichat --role code -c echo server in rust
aichat --role shell -e install nvim

2. add roles to roles.yaml

• add a role named __execute__ to override the execute role.
• add a role named __code__ to override the code role.