refactor(create-expert): document integration-manager design rationale

[FL4TLiN3]

Summary

Adds explicit Design Rationale section to integration-manager explaining why multiple responsibilities are appropriate for this expert.

Added Design Rationale

## Design Rationale

This expert has multiple responsibilities (orchestration, trade-off analysis, scoring)
because they are all "integration" concerns - they require seeing both functional and
usability results together. Splitting these would require passing combined context
between more experts, adding complexity without benefit.

Why Keep Current Design

1. All responsibilities are integration concerns - they need combined functional + usability results
2. Splitting adds complexity - would require passing combined context between more experts
3. Trade-off analysis needs both contexts - can't analyze security vs usability without both
4. Scoring requires holistic view - needs all test results to calculate overall quality

Additional Changes

• Refactored to use declarative style (matching other PRs in this series)
• Replaced numbered workflow with "Testing Strategy" and "Responsibilities" sections
• Added "Delegates" section for consistency

Closes #383

Test plan

• CI passes
• No behavior changes expected

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Note

Integration-manager instruction updates

• Adds explicit ## Design Rationale explaining why orchestration, trade-off analysis, and scoring stay within one expert (integration concerns requiring combined context)
• Clarifies wording in Your Role and Testing Strategy (parallel delegation; holistic assessment) without changing delegates or functionality

Release note

• Adds changeset for a patch release

^{Written by Cursor Bugbot for commit aab1396. This will update automatically on new commits. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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