Add support-case investigation workflow

[mcgonago]

Summary

• Add a second workflow (support-case) for investigating OpenStack Horizon customer support cases
• New agent persona: Support Case Investigator (RHOSP upgrades, RBAC/policy, cross-service dependencies)
• New knowledge layer: symptom classification taxonomy, API proxy code tracing, version regression detection (RHOSP 13→18.0), oslo.policy analysis, must-gather/SOSreport patterns
• Skill: /support-case-investigation <case-number>
• Cursor integration: agent + rules symlinks

What it does

Given a support case number or symptom description, the workflow:

1. Classifies the symptom (API failure, RBAC, upgrade regression, UI rendering, session/auth, performance)
2. Traces the Horizon code path from UI action to backend API
3. Compares code between RHOSP versions if upgrade-related
4. Analyzes policy rules if authorization-related
5. Produces three artifacts: investigation_report.md, customer_response.md, code_trace.md

Test plan

• Ran /support-case-investigation 04426889 end-to-end
• Verified all three artifacts produced with correct structure
• Review knowledge base accuracy against upstream Horizon source

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