feat: Add CLAUDE.md and Cursor rules development guidance

.claude/settings.local.json

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+{
+  "permissions": {
+    "allow": [
+      "Bash(find:*)",
+      "Bash(ls:*)",
+      "Bash(git:*)",
+      "Bash(yarn:*)"
+    ],
+    "deny": []
+  }
+}

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Development Commands
+
+### Build Commands
+- `yarn build` - Full production build with package verification
+- `yarn build:dev` - Development build (transpile + types)
+- `yarn build:dev:watch` - Development build in watch mode (recommended for development)
+- `yarn build:dev:filter <package>` - Build specific package and its dependencies
+- `yarn build:types:watch` - Watch mode for TypeScript types only
+- `yarn build:bundle` - Build browser bundles only
+
+### Testing
+- `yarn test` - Run all tests (excludes integration tests)
+- `yarn test:unit` - Run unit tests only
+- `yarn test:pr` - Run tests affected by changes (CI mode)
+- `yarn test:pr:browser` - Run affected browser-specific tests
+- `yarn test:pr:node` - Run affected Node.js-specific tests
+
+### Linting and Formatting
+- `yarn lint` - Run ESLint and Prettier checks
+- `yarn fix` - Auto-fix linting and formatting issues
+- `yarn lint:es-compatibility` - Check ES compatibility
+
+### Package Management
+- `yarn clean` - Clean build artifacts and caches
+- `yarn clean:deps` - Clean and reinstall all dependencies
+
+## Repository Architecture
+
+This is a Lerna monorepo containing 40+ packages in the `@sentry/*` namespace. Key architectural components:
+
+### Core Packages
+- `packages/core/` - Base SDK with interfaces, type definitions, and core functionality
+- `packages/types/` - Shared TypeScript type definitions
+- `packages/browser-utils/` - Browser-specific utilities and instrumentation
+
+### Platform SDKs
+- `packages/browser/` - Browser SDK with bundled variants
+- `packages/node/` - Node.js SDK with server-side integrations
+- `packages/bun/`, `packages/deno/`, `packages/cloudflare/` - Runtime-specific SDKs
+
+### Framework Integrations
+- Framework packages follow naming: `packages/{framework}/` (react, vue, angular, etc.)
+- Each has client/server entry points where applicable (e.g., nextjs, nuxt, sveltekit)
+
+### Build System
+- Uses Rollup for bundling with config files: `rollup.*.config.mjs`
+- TypeScript with multiple tsconfig files per package (main, test, types)
+- Lerna manages package dependencies and publishing
+- Vite for testing with `vitest`
+
+### Package Structure Pattern
+Each package typically contains:
+- `src/index.ts` - Main entry point
+- `src/sdk.ts` - SDK initialization logic
+- `rollup.npm.config.mjs` - Build configuration
+- `tsconfig.json`, `tsconfig.test.json`, `tsconfig.types.json` - TypeScript configs
+- `test/` directory with corresponding test files
+
+### Key Development Notes
+- Uses Volta for Node.js/Yarn version management
+- Requires initial `yarn build` after `yarn install` for TypeScript linking
+- Integration tests are in separate packages (`dev-packages/`)
+- Native profiling requires Python <3.12 for binary builds
+- Bundle outputs vary - check `build/bundles/` for specific files after builds
+
+## Git Flow Branching Strategy
+
+This repository uses **Git Flow** branching model:
+
+### Branch Structure
+- `master` - Production releases only
+- `develop` - Main development branch (target for PRs)  
+- `feat/` - Feature branches (e.g., `feat/add-feature`)
+- `release/X.Y.Z` - Release preparation branches
+
+### Workflow
+- **All PRs target `develop` branch** (not `master`)
+- Features are merged: `feat/branch` → `develop`
+- Releases are created: `develop` → `release/X.Y.Z` → `master`
+- Automated workflow syncs `master` back to `develop` after releases
+
+### Branch Naming
+- Features: `feat/descriptive-name`
+- Releases: `release/X.Y.Z`
+
+## Code Quality Requirements
+
+**CRITICAL**: This is a production SDK used by thousands of applications. All changes must be:
+
+### Mandatory Checks
+- **Always run `yarn lint`** - Fix all linting issues before committing
+- **Always run `yarn test`** - Ensure all tests pass
+- **Run `yarn build`** - Verify build succeeds without errors
+
+### Before Any Commit
+1. `yarn lint` - Check and fix ESLint/Prettier issues
+2. `yarn test` - Run relevant tests for your changes
+3. `yarn build:dev` - Verify TypeScript compilation
+
+### CI/CD Integration
+- All PRs automatically run full lint/test/build pipeline
+- Failed checks block merging
+- Use `yarn test:pr` for testing only affected changes
+
+## Testing Single Packages
+To test a specific package: `cd packages/{package-name} && yarn test`
+To build a specific package: `yarn build:dev:filter @sentry/{package-name}`