Add workflow decomposition support

[dearrudam]

This pull request introduces comprehensive support for workflow decomposition in the SLDD skill, enabling large ideas to be broken down into coordinated but independently executable workflows. The changes formalize the requirements, technical design, and acceptance criteria for a new workflow-set workflow kind, while preserving backward compatibility and existing feature workflow behavior.

Key changes include:

Feature and Workflow Decomposition Behavior:

• Adds a new workflow-set workflow kind with dedicated parent steps (01-workflow-set-plan, 02-scaffold-children, 03-verify-workflow-set) to coordinate decomposition and child workflow scaffolding. Existing workflows without kind are treated as feature for backward compatibility. [1] [2]
• Implements explicit approval and materialization modes for workflow-set planning, ensuring that no child workflows are created or overwritten without user consent. [1] [2]
• Enforces predecessor completion gates: child workflows scaffolded from a workflow-set cannot proceed until all predecessor workflows have completed verification. [1] [2]

Technical Design and Data Flow:

• Documents the full requirements traceability, architecture, component responsibilities, and data flows for the new decomposition feature, ensuring clarity for both users and maintainers.
• Details security, observability, and guardrail requirements to prevent accidental overwrites, unsafe execution, or loss of backward compatibility.

Acceptance Criteria and Out-of-Scope Clarification:

• Provides exhaustive acceptance criteria and explicitly lists out-of-scope items to guide implementation and review, ensuring the change remains focused and safe for first-version rollout.

These changes collectively enable SLDD to safely and transparently decompose large initiatives, improving scalability and maintainability for both human and AI users.