feat: core brain upgrade v3.0 — shell-core separation architecture
- Created src/brain/ module (prompt-assembler, mode-detector, model-router, context-trimmer, memory-manager)
- Added HLI BRAIN domain endpoints (prompt, route, context, memory, status)
- Upgraded api-proxy.js with auto-fallback across providers
- Unified brain version to v3.0 across all files
- Frontend now calls backend brain APIs with local fallback
- Added truth-source federation documentation
Co-authored-by: qinfendebingshuo <207279273+qinfendebingshuo@users.noreply.github.com>
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