Add project-level scope assessment to brainstorming pipeline

[obra]

Summary

Stacked on #441 (architecture guidance).

• Brainstorming now assesses scope as the first step: if a request describes multiple independent subsystems, it flags this immediately and helps decompose into sub-projects before asking detailed design questions
• Spec reviewer checks that specs are focused enough for a single plan
• Writing-plans has a backstop scope check if brainstorming missed it

Testing

Ran 5 end-to-end scenarios with claude -p --plugin-dir:

Scenario	Expected	Result
Small greenfield CLI tool	No scope flag	PASS
Brownfield messy (monolithic server.js)	Flag architecture	PARTIAL (pipe-mode limitation)
Brownfield clean (well-structured API)	Follow existing patterns	PASS
Large single system (Rust markdown editor)	No false decomposition	PASS
Massive multi-system (Salesforce competitor)	Strong scope decomposition	PASS

Initial version had scope assessment as a separate step after "Understanding the idea" — model skipped it. Fixed by inlining as first bullet in Understanding phase. Retest confirmed.

Test plan

• Large scope triggers decomposition (Salesforce competitor test)
• Small scope does not over-decompose (CLI tool test)
• Large single system not falsely decomposed (markdown editor test)
• Scope assessment fires before clarifying questions (v2 retest)

[coderabbitai]

Important

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch wip/project-decomposition

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