chore: add AGENTS.md and NixOS dev environment

AGENTS.md

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+# AGENTS.md
+
+This file provides guidance to AI coding agents when working with code in this repository.
+
+## Project Overview
+
+OpenWPM is a web privacy measurement framework for conducting large-scale privacy studies (thousands to millions of websites). Built on Firefox with Selenium automation, it captures HTTP traffic, JavaScript API calls, cookies, navigation, and DNS queries through a privileged WebExtension.
+
+## Build & Development Commands
+
+### Initial Setup
+```bash
+./install.sh  # Creates conda env, installs Firefox, builds extension
+```
+
+### Running Tests
+```bash
+# Run all tests
+pytest
+
+# Run specific test file
+pytest test/test_storage.py -v
+
+# Run tests matching pattern
+pytest -k "test_http" -v
+
+# Run tests by marker
+pytest -m pyonly  # Python-only tests (no browser)
+pytest -m slow    # Slow tests
+```
+
+### Linting & Formatting
+
+**Python:**
+```bash
+pre-commit run --all-files  # Run all hooks
+black .                      # Format Python
+isort .                      # Sort imports
+mypy openwpm                 # Type checking
+```
+
+**Extension (from Extension/ directory):**
+```bash
+npm run lint    # ESLint + Prettier + web-ext lint
+npm run fix     # Auto-fix issues
+npm run build   # Rebuild extension (TypeScript → webpack → web-ext)
+```
+
+### Rebuilding Extension
+```bash
+scripts/build-extension.sh
+# Or from Extension/: npm run build
+```
+
+### Updating Dependencies
+```bash
+scripts/repin.sh  # Don't edit environment.yaml directly
+```
+
+## Architecture
+
+```
+TaskManager (orchestrator)
+├── BrowserManagerHandle[] → BrowserManager (per-browser process)
+│                              └── Selenium WebDriver → Firefox + WebExtension
+└── StorageController (isolated process)
+    ├── StructuredStorageProvider (SQLite/Parquet/S3/GCS)
+    └── UnstructuredStorageProvider (LevelDB/Gzip/S3/GCS)
+```
+
+### Core Components
+
+- **TaskManager** (`openwpm/task_manager.py`): Orchestrates browsers, manages command queues, runs watchdogs for crash recovery
+- **BrowserManager** (`openwpm/browser_manager.py`): Wraps Selenium, executes commands, handles browser lifecycle
+- **WebExtension** (`Extension/src/`): TypeScript extension collecting HTTP, cookies, JS calls, navigation via privileged browser APIs
+- **Storage System** (`openwpm/storage/`): Receives data from extension via sockets, writes to configured backends
+- **Commands** (`openwpm/commands/`): Extend `BaseCommand` with `execute()` method
+
+### Configuration
+
+- `ManagerParams`: Platform settings (num_browsers, data_directory, log_path)
+- `BrowserParams`: Per-browser settings (instrumentation flags, display mode, profile handling)
+- Both defined in `openwpm/config.py` with validation functions
+- See [docs/Configuration.md](docs/Configuration.md) for full details
+
+### Known Issues
+
+- `callstack_instrument` is broken — enabling it raises `ConfigError`. See [#557](https://github.com/openwpm/OpenWPM/issues/557).
+
+### Data Schema
+
+Schema files must be kept in sync:
+- `openwpm/storage/schema.sql` (SQLite)
+- `openwpm/storage/parquet_schema.py` (Parquet)
+
+## Scratch Space
+
+Use `datadir/` (project root) for any temporary or scratch data — crawl outputs, test databases, logs, etc. This directory is gitignored and is the conventional location for local data. Do not use `/tmp`, `~`, or other locations outside the project.
+
+`demo.py` defaults to writing here (`manager_params.data_directory = Path("./datadir/")`). The directory is created on demand.
+
+## Release Process
+
+When creating releases, PRs, or interacting with GitHub, prefer using the `gh` CLI over the web UI.
+
+## Key Files
+
+- `demo.py`: Reference implementation showing typical usage
+- `custom_command.py`: Example of custom command implementation
+- `test/manual_test.py`: Interactive debugging (`python -m test.manual_test --selenium`)
+
+## Display Modes
+
+- `native`: GUI visible (default)
+- `headless`: Firefox headless (no X server needed)
+- `xvfb`: X virtual framebuffer (full browser, no GUI, for servers)

nix/README.md

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+# Nix Development Environment
+
+**This is only intended for NixOS users.** If you are on a standard Linux distribution or macOS, follow the regular setup instructions in the project root README instead.
+
+NixOS cannot run the pre-built Firefox binaries that OpenWPM downloads without an FHS-compatible environment. The `shell.nix` in this directory provides that environment.
+
+## Usage
+
+```sh
+nix-shell nix/shell.nix
+```
+
+This drops you into an FHS environment with the `openwpm` conda env already activated.

nix/shell.nix

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+{ pkgs ? import <nixpkgs> {} }:
+
+let
+  packages = with pkgs; [
+    # Build essentials
+    stdenv.cc.cc.lib
+    gcc
+    gnumake
+    pkg-config
+    zlib
+    openssl
+
+    # Python build dependencies
+    libffi
+    readline
+    ncurses
+    bzip2
+    xz
+    sqlite
+
+    # Firefox dependencies
+    gtk3
+    glib
+    atk
+    gdk-pixbuf
+    pciutils
+    dbus-glib
+    libGL
+    libGLU
+    alsa-lib
+    libpulseaudio
+    ffmpeg
+    pango
+    cairo
+    freetype
+    fontconfig
+
+    # X11 libraries
+    libx11
+    libxext
+    libxrender
+    libxtst
+    libxi
+    libxcomposite
+    libxcursor
+    libxdamage
+    libxfixes
+    libxrandr
+    libxcb
+    xvfb
+
+    # Utilities
+    git
+    which
+    curl
+    wget
+    file
+    gnugrep
+    coreutils
+    bashInteractive
+
+    # Node.js for extension build
+    nodejs_22
+  ];
+
+  libraryPath = pkgs.lib.makeLibraryPath (with pkgs; [
+    stdenv.cc.cc.lib
+    zlib
+    libGL
+    glib
+    gtk3
+    atk
+    gdk-pixbuf
+    pciutils
+    alsa-lib
+    libpulseaudio
+    libx11
+    libxext
+    libxrender
+    libxtst
+    libxcomposite
+    libxcursor
+    libxdamage
+    libxfixes
+    libxrandr
+    dbus-glib
+    pango
+    cairo
+    freetype
+    fontconfig
+  ]);
+
+  fhsEnv = pkgs.buildFHSEnv {
+    name = "openwpm";
+    targetPkgs = _: packages;
+    profile = ''
+      export LD_LIBRARY_PATH="${libraryPath}:$LD_LIBRARY_PATH"
+      export PATH="$HOME/.conda/bin:$PATH"
+      source "$HOME/.conda/etc/profile.d/conda.sh"
+    '';
+    runScript = let
+      condaInit = pkgs.writeText "conda-init.sh" ''
+        set -h  # Enable command hashing
+        source "$HOME/.conda/etc/profile.d/conda.sh"
+        conda activate openwpm
+      '';
+    in pkgs.writeShellScript "openwpm-shell" ''
+      export BASH_ENV="${condaInit}"
+      exec bash --rcfile "${condaInit}" "$@"
+    '';
+  };
+
+in pkgs.mkShell {
+  packages = [ fhsEnv ];
+  shellHook = ''
+    exec openwpm
+  '';
+}