Add AGENTS.md file (#7557)

Author: ggdouglas

AGENTS.md

@@ -0,0 +1,29 @@
+# Blueprint.js Agentic Development Guide
+
+## Build/Test Commands
+
+-   **Build**: `yarn compile` (all packages), `yarn nx compile @blueprintjs/core` (single package)
+-   **Test all**: `yarn test`, `nx run-many -t test`
+-   **Test single package**: `yarn nx test:karma @blueprintjs/core` or `cd packages/core && yarn test:karma`
+-   **Distribute**: `yarn dist`, `yarn nx dist @blueprintjs/core` (single package)
+-   **Lint**: `yarn lint`, `yarn lint-fix` (auto-fix), `yarn nx lint @blueprintjs/core` (single package)
+-   **Format**: `yarn format`, `yarn format-check`
+-   **Verify all**: `yarn verify` (compile + dist + test + lint + format-check)
+
+## Architecture
+
+-   **Monorepo** using Yarn workspaces + Nx task runner
+-   **Packages**: Core UI components in `packages/` - core, datetime, select, table, icons, colors
+-   **Apps**: docs-app (blueprintjs.com), demo-app, table-dev-app for development
+-   **Build tools**: karma-build-scripts, node-build-scripts, webpack-build-scripts
+-   **Testing**: Mocha + Karma (components), Jest (build scripts), Enzyme + Chai + Sinon
+    -   New tests should be written with React Testing Library (RTL)
+
+## Code Style
+
+-   **Prettier**: 120 char width, 4-space tabs (2 for SCSS/YAML), trailing commas
+-   **ESLint**: TypeScript-ESLint + custom @blueprintjs rules, no console.log
+-   **Imports**: Use workspace: deps, no lodash-es (use lodash submodules)
+-   **Types**: Strict TypeScript, React 18 peer deps
+-   **Components**: Follow existing patterns in packages/core/src/components/
+-   **Styling**: SCSS in src/, compiled to lib/css/