docs: feature inventory and implementation guide for architects

[rahuliitk]

Summary

• openlintelfeatures.md: Complete inventory of all 42 existing features + 80+ missing features identified from the perspective of award-winning architects, home builders, and interior designers
• missingfeaturesimplementation.md: Detailed step-by-step implementation guide for every missing feature — includes database schemas (Drizzle ORM), tRPC routers, Python service agents (LangGraph), frontend pages (Next.js 14), and Docker configuration — all aligned with the existing codebase architecture
• 24-month phased roadmap prioritizing features by professional impact: 2D editor, kitchen/bath module, RFI management, client portal in Phase 1 through marketplace and AI learning in Phase 6

Test plan

• Verify markdown renders correctly on GitHub
• Review feature inventory for completeness against codebase
• Review implementation guide for alignment with existing architecture patterns

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