Rename agent_plan feature flag to workplan

What does this MR do?

Renames the WIP feature flag agent_plan to workplan and updates all feature flag usages in the backend.

Changes

• Renamed ee/config/feature_flags/wip/agent_plan.yml to workplan.yml and updated the name and description.
• Updated the Feature.disabled?(:agent_plan, ...) checks to :workplan in:
  • ee/app/models/ee/work_items/types_framework/system_defined/type.rb
  • ee/app/models/work_items/types_framework/custom/type.rb
• Updated stub_feature_flags(agent_plan:) to workplan: in the affected specs.

Out of scope

Backend constants are intentionally not renamed (model WorkItems::AgentPlan, widget type 'agent_plan', DB table work_item_agent_plans, backup task agent_plan_content, and :agent_plan associations). Only the feature flag was renamed.