Add project-level scope assessment to brainstorming pipeline

Brainstorming now assesses whether a project is too large for a single
spec and helps decompose into sub-projects. Scope check is inline in
the understanding phase (testing showed it was skipped as a separate step).
Spec reviewer also checks scope. Writing-plans has a backstop.

skills/brainstorming/SKILL.md

@@ -63,7 +63,9 @@ digraph brainstorming {
 **Understanding the idea:**
 
 - Check out the current project state first (files, docs, recent commits)
-- Ask questions one at a time to refine the idea
+- Before asking detailed questions, assess scope: if the request describes multiple independent subsystems (e.g., "build a platform with chat, file storage, billing, and analytics"), flag this immediately. Don't spend questions refining details of a project that needs to be decomposed first.
+- If the project is too large for a single spec, help the user decompose into sub-projects: what are the independent pieces, how do they relate, what order should they be built? Then brainstorm the first sub-project through the normal design flow. Each sub-project gets its own spec → plan → implementation cycle.
+- For appropriately-scoped projects, ask questions one at a time to refine the idea
 - Prefer multiple choice questions when possible, but open-ended is fine too
 - Only one question per message - if a topic needs more exploration, break it into multiple questions
 - Focus on understanding: purpose, constraints, success criteria

skills/brainstorming/spec-document-reviewer-prompt.md

@@ -23,6 +23,7 @@ Task tool (general-purpose):
     | Consistency | Internal contradictions, conflicting requirements |
     | Clarity | Ambiguous requirements |
     | YAGNI | Unrequested features, over-engineering |
+    | Scope | Focused enough for a single plan — not covering multiple independent subsystems |
     | Architecture | Units with clear boundaries, well-defined interfaces, independently understandable and testable |
 
     ## CRITICAL