chore: add .github/copilot-instructions.md with repository guidance

Co-authored-by: atherdon <1469198+atherdon@users.noreply.github.com>

Author: Copilot

.github/copilot-instructions.md

@@ -0,0 +1,68 @@
+# Copilot Instructions for markdown-regex
+
+## Project Overview
+
+`markdown-regex` is a lightweight JavaScript library (~2 KB, zero runtime dependencies) that exports named RegExp constants for parsing and extracting elements from Markdown content. It supports CommonJS, ES Modules, and IIFE browser builds, and includes TypeScript definitions.
+
+## Repository Structure
+
+```
+src/
+  index.js          # Main entry point – re-exports all patterns
+  index.d.ts        # TypeScript type definitions
+  tags/             # Regex patterns for inline/block markdown tags
+  lists/            # Regex patterns for ordered and unordered lists
+  custom/           # Custom patterns
+  chat/             # Chat-related patterns
+  utils.js
+tests/              # Jest test files mirroring the src/ layout
+dist/               # Build output (generated, do not edit)
+```
+
+## Build, Test, and Lint Commands
+
+| Purpose | Command |
+|---|---|
+| Build (clean + bundle) | `npm run build` |
+| Run tests | `npm test` |
+| Lint (check) | `npm run lint` |
+| Lint (auto-fix) | `npm run lint:fix` |
+| Format with Prettier | `npm run lint:prettier` |
+
+Always run `npm test` and `npm run lint` before submitting a PR. Both must pass.
+
+## Adding a New Regex Pattern
+
+1. Create a new file under `src/tags/` or `src/lists/` (e.g., `src/tags/my-pattern.js`) that exports a named `REGEXP_*` constant.
+2. Re-export the constant from the appropriate barrel file (`src/tags/index.js` or `src/lists/index.js`).
+3. Re-export it from `src/index.js`.
+4. Add the TypeScript declaration to `src/index.d.ts`.
+5. Add a corresponding test file under `tests/tags/` or `tests/lists/` following the existing test conventions.
+6. Update `readme.md` to document the new constant.
+7. Add an entry to `CHANGELOG.md` under `[Unreleased]`.
+
+## Code Style
+
+- **Language**: ES Modules (`export`/`import`). Rollup handles CommonJS output.
+- **Naming**: All exported regex constants must be named `REGEXP_*` in `SCREAMING_SNAKE_CASE`.
+- **Line length**: Maximum 120 characters (enforced by ESLint).
+- **Camel case**: Use camelCase for non-constant identifiers.
+- **No unnecessary escapes**: The linter allows `no-useless-escape` and `no-control-regex` to be off for regex patterns.
+- **Formatting**: Run `npm run lint:prettier` to auto-format source files.
+
+## Testing
+
+- Tests use **Jest 29** with ES module support (`NODE_OPTIONS=--experimental-vm-modules`).
+- Test files live in `tests/` and mirror the `src/` directory structure.
+- Coverage threshold is **70%** for branches, functions, lines, and statements.
+- Each regex pattern should have tests for both **matching** and **non-matching** cases, and cover edge cases such as Windows (`\r\n`) vs Unix (`\n`) line endings.
+
+## TypeScript
+
+When adding or modifying exported constants, always update `src/index.d.ts` to keep the TypeScript definitions in sync.
+
+## Pull Request Guidelines
+
+- Reference the related GitHub issue in the PR description (`Closes #<issue>`).
+- Ensure all checklist items in `.github/PULL_REQUEST_TEMPLATE.md` are completed.
+- Keep changes focused; one logical change per PR.