Expose image picker and sample actions to the agent

[cgasgarth]

Goal

Expose supported “pick from image” / sample-based actions so the agent can use picker workflows directly.

Current limitation

Picker tools are currently skipped because they are not simple param-backed widgets.

Examples

• iop/temperature/settings/from image area
• iop/rgblevels/pickers/black
• iop/rgblevels/pickers/gray
• iop/rgblevels/pickers/white

Starting point

• darktable/src/common/agent_catalog.c:118
• darktable/src/common/agent_catalog.c:259

Notes

• This probably needs an explicit picker action model plus coordinates/selection handling.
• Define what image-space input the agent can provide and how failures are reported.