Add AGENTS.md with repository instructions

Add comprehensive instructions for working with this compiler project, including key commands, architecture overview, dependencies, and environment setup information.

AGENTS.md

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+# Jompiler Agent Instructions
+
+This is a C compiler that targets x86-64 ELF object files.
+
+## Key Commands
+
+- `node index.js input.c` - Compile and link input.c to produce a.out
+- `node index.js input.c && ./a.out` - Compile, link, and run the compiled program
+
+## Architecture
+
+This is a single-file compiler written in JavaScript with the following phases:
+1. Lexing (tokenization) - `lex()` function
+2. Parsing (AST construction) - `parse()` function  
+3. Lowering (x86-64 code generation) - `lower()` function
+4. Linking - `link()` function that shells out to `gcc`
+
+## Dependencies
+
+The system requires:
+- Node.js (tested with Node.js 22)
+- GCC (for linking)
+- LLVM tools (for development environment - installed via nix)
+
+## Important Details
+
+- The compiler supports basic C constructs: integers, variables, basic arithmetic, function declarations, function calls, returns, and basic control flow
+- Uses the standard C calling convention for x86-64
+- Generates x86-64 ELF object files
+- Has no support for complex C features like pointers, conditionals, loops, arrays
+- Supports the same C standard as referenced in the comment: C11 (ISO/IEC 9899:2011)
+- Has a specific test case in `input.c` that demonstrates basic functionality
+- The `dump-main.gdb` file is used for debugging and disassembly
+
+## Runtime Environment
+
+- Uses Nix for environment setup: `nix-shell` or `direnv` with the provided `shell.nix`
+- Requires `node` to be available from PATH
+- Requires `gcc` and `gdb` for linking and debugging
+- Uses `spawn` to execute external commands (GCC linking, GDB debugging)