Design an AI Software Engineer

Design a complete AI Software Engineer system that operates at three levels: inline autocomplete in 300ms (serving 100M completions/day), a codebase-aware agent that refactors across 12 files in 45 seconds, and a fully autonomous builder that takes a natural language spec, asks clarifying questions, designs the architecture, scaffolds the project, builds each module with error recovery, responds to feedback over hours, and deploys to production. Cover the local context engine (tree-sitter AST, dependency graphs, LSP, git), multi-model routing, agent tool system with sandbox execution, long-running checkpointing and crash recovery, persistent project memory, multi-agent orchestration, and the economics of serving 1M developers at $20/month.