feat: support agent variables

[sigoden]

define variables

Variables are generally used to record a certain behavior or preference of a user.

variables:
  - name: foo
    description: This is a foo
  - name: bar
    description: This is a bar with default value
    default: val

Variables can be used in the instructions.

instructions: |
  The instructions can inline {{foo}} and {{bar}} variables.

init variables

save variables

/agents//variables.yaml

foo: v1
bar: v2