docs: add AGENTS.md with development workflow

AGENTS.md

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+# Keystone AI Agent Instructions
+
+## Project Overview
+
+Keystone is an Electron desktop app for AI-assisted software architecture documentation. Built with React 19, TypeScript 5.7, Tailwind 3.4, electron-vite 5, Zustand 5, better-sqlite3 11, and tRPC 10.
+
+## Architecture
+
+- `src/main/` - Electron main process (services, database, IPC)
+- `src/renderer/` - React UI (components, features, stores)
+- `src/agents/` - AI orchestration (providers, specialists, orchestrator)
+- `src/shared/` - Types, utilities, constants
+- `src/preload/` - Electron preload scripts
+
+## Development Workflow
+
+### Commit and Push Regularly
+
+- Commit changes in logical units of work
+- Push frequently to keep remote in sync
+- Use conventional commit format: `feat`, `fix`, `docs`, `chore`, `refactor`, `test`
+
+### Branch Strategy
+
+- Main branch is protected; changes require pull requests
+- Create feature branches: `feat/<description>`, `fix/<description>`
+- Use squash merge for clean history
+
+### Auto-Merge When Work is Completed
+
+- When a feature is complete and tested, merge the PR immediately
+- Use `gh pr merge --squash --auto` for auto-merge with squash
+- Don't leave PRs open unnecessarily
+
+### Before Committing
+
+1. Run `pnpm typecheck` to verify types
+2. Run `pnpm lint` to check for linting issues
+3. Run `pnpm test` for unit tests (if applicable)
+
+## Key Technical Notes
+
+### electron-vite Configuration
+
+- electron-vite v5.0.0+ required for Vite 7 compatibility
+- `tsconfig.web.json` must include `src/main/**/*` and `src/agents/**/*` because renderer imports `AppRouter` type
+
+### OAuth Authentication
+
+- OpenAI OAuth requires fixed port 1455 and callback path `/auth/callback`
+- Anthropic does NOT support OAuth (API keys only, per TOS)
+- Google OAuth is experimental (access may be revoked)
+
+### tRPC Context
+
+- `createContext()` must be async (`Promise<Context>`)
+- Services are instantiated once and cached in `_context`