Add workflow decomposition support

[dearrudam]

This pull request introduces the concept of workflow decomposition to the SLDD skill, enabling large ideas to be split into multiple coordinated feature workflows through a new "workflow-set" mechanism. It updates documentation, schema, and navigation logic to support workflow-sets, adds new parent workflow steps, and ensures backward compatibility with existing feature workflows.

Major features and changes:

Workflow-set support and process changes

• Introduced the workflow-set workflow kind, allowing large ideas to be decomposed into coordinated child feature workflows. Added parent steps (01-workflow-set-plan, 02-scaffold-children, 03-verify-workflow-set) and clarified their flow, approval, and state management in both README.md and SKILL.md. [1] [2] [3] [4] [5]
• Updated navigation and resume logic to handle workflow-sets: parent steps are completed before child workflows are actionable, child progress is read-only in the parent, and journals without kind are treated as normal feature workflows for compatibility. [1] [2]

Schema and journal updates

• Extended _spec-journal.schema.json to support kind: "workflow-set", new parent workflow steps, child workflow metadata, relationships, and scaffold state tracking. Added validation for new fields and states. [1] [2] [3]

Documentation and guardrails

• Documented workflow-set usage, states, child workflow requirements, and explicit user approval for decomposition in README.md, SKILL.md, and step files. Clarified that workflow-set parents do not persist child execution progress or auto-select children. [1] [2] [3] [4] [5]
• Added rules and anti-patterns to AGENTS.md to prevent premature design, binding requirements, or treating workflow-set context as satisfying feature workflow gates. [1] [2]

Step logic enhancements

• In Step 01, recommend workflow-set planning for large ideas and enforce predecessor verification for scaffolded children.

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These changes enable SLDD to safely manage large, multi-part features by coordinating multiple child workflows under an approved parent plan, while maintaining strict gating, explicit user approval, and backward compatibility.