Set up Copilot instructions (#42)

* Initial plan

* Add .github/copilot-instructions.md with repository-specific guidance

Co-authored-by: dmmulroy <2755722+dmmulroy@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: dmmulroy <2755722+dmmulroy@users.noreply.github.com>

Author: Copilot

.github/copilot-instructions.md

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+# Copilot Instructions for ts-error-translator.nvim
+
+This is a Neovim plugin written in Lua that translates TypeScript error messages into human-readable explanations. It's a port of [Matt Pocock's ts-error-translator for VSCode](https://github.com/mattpocock/ts-error-translator).
+
+## Project Structure
+
+```
+lua/ts-error-translator/
+  init.lua         -- Public API and plugin setup
+  parser.lua       -- Core parsing logic (extracts TS codes, O(1) lookup)
+  matcher.lua      -- Pattern matching with vim.regex
+  utils.lua        -- Helper functions
+  lru.lua          -- LRU cache implementation
+  diagnostic.lua   -- vim.diagnostic integration
+  db.lua           -- Error database indexed by code (auto-generated)
+```
+
+## Code Style
+
+- Follow the [StyLua](https://github.com/JohnnyMorganz/StyLua) formatter configuration in `.stylua.toml`
+- Use 2 spaces for indentation
+- Maximum line width of 120 characters
+- Use double quotes for strings
+- Add type annotations using `@class`, `@field`, `@param`, `@return` comments for public APIs
+- Include `---@diagnostic disable:` comments when necessary to suppress known false positives
+
+## Building
+
+The error database (`lua/ts-error-translator/db.lua`) is auto-generated from:
+- `tsErrorMessages.json` (TypeScript error patterns)
+- `errors/*.md` (improved human-readable messages)
+
+To rebuild the database:
+```bash
+make build
+# or: node build-lua-db.js
+```
+
+**Important**: Never manually edit `lua/ts-error-translator/db.lua` - it's generated code.
+
+## Testing
+
+Tests use [plenary.nvim](https://github.com/nvim-lua/plenary.nvim) test framework.
+
+Run all tests:
+```bash
+make test
+```
+
+Run a specific test file:
+```bash
+make test-file FILE=tests/spec/parser_spec.lua
+```
+
+Tests are located in `tests/spec/` and follow the pattern `*_spec.lua`.
+
+## Key Conventions
+
+- The plugin uses O(1) lookup via error codes extracted from messages
+- Error codes are extracted using the pattern `[Tt][Ss](%d+)`
+- The LRU cache improves performance for repeated lookups
+- Public API functions should be exposed through `init.lua`
+- Use `pcall` for error handling to provide graceful fallbacks
+
+## Adding New Error Translations
+
+1. Create a new markdown file in `errors/` named `{error_code}.md`
+2. Include frontmatter and the improved message body
+3. Run `make build` to regenerate `db.lua`
+
+## Dependencies
+
+- **Runtime**: No external dependencies (uses native vim.regex)
+- **Build**: Node.js with `front-matter` package
+- **Testing**: plenary.nvim