Add skill selection layer with constrained choices (no free-form skills)

[coderooz]

Agents must select skills from a predefined registry instead of defining arbitrary capabilities.

Goals:

• prevent unnecessary or irrelevant skill usage
• reduce prompt/context noise
• improve execution precision

Features:

• dropdown/selection-based skill assignment
• validation against registry
• optional skill activation per task/session

Behavior:

• agent chooses only relevant skills
• system restricts execution scope to selected skills