Add readme and claude.me

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.
+
+## Project Overview
+
+Custom QMK firmware for a System76 Launch 3 keyboard (RP2040, 84-key). The custom keymap adds OS-aware shortcut translation (Linux/Mac/Windows) with automatic OS detection.
+
+## Build Commands
+
+```bash
+# Development (uses symlink to custom keymap)
+nix develop
+qmk compile -kb system76/launch_3 -km custom
+
+# Production build (sandboxed, outputs result/system76_launch_3_custom.uf2)
+nix build
+```
+
+New files in `keymaps/custom/` must be `git add`ed before `nix build` — flakes only see git-tracked files.
+
+## Architecture
+
+### Repository Layout
+
+- `qmk_firmware/` — System76's QMK fork (git submodule, do not modify)
+- `keymaps/custom/` — Custom keymap, kept separate from the submodule
+- `flake.nix` — Dev shell and sandboxed firmware build; uses `self.submodules = true`
+
+### Custom Keymap Files (`keymaps/custom/`)
+
+- `keymap.c` — Keymaps, layer override, modifier rewriting, process_record_user
+- `os_detect.c` — OS auto-detection polling, os_mode/os_mode_manual definitions, RGB flash
+- `os_mode.h` — Shared `os_mode_t` enum and extern declarations
+- `rules.mk` — Enables OS_DETECTION and adds `SRC += os_detect.c`
+
+### Key Constraints
+
+**keymap_introspection.c textually `#include`s keymap.c** — this means:
+- Headers in `keymaps/custom/` can't be `#include`d from `keymap.c` (the compiler resolves includes relative to `quantum/keymap_introspection.c`, not the included file)
+- Variables shared between `keymap.c` and other files must be non-static with `extern` declarations
+- Guard OS-specific code with `#ifdef OS_DETECTION_ENABLE` if it uses types from guarded headers, or put it in a separate `.c` file (preferred)
+
+**keyboard_post_init_user is taken** by `launch_3.c` — use `matrix_init_user` instead.
+
+**DYNAMIC_KEYMAP_ENABLE** causes EEPROM to override PROGMEM keymaps — `dynamic_keymap_reset()` in `matrix_init_user` forces PROGMEM back.
+
+### Modifier Rewriting System
+
+`mod_rewrite_t` structs define from→to modifier translations. `normalize_mods()` collapses left/right modifier variants into a single nibble for comparison. `mods_match()` does exact mod matching and skips rewrites when FN layer is active.
+
+### OS Mode
+
+`os_mode_t` enum (NONE/LINUX/MAC/WINDOWS). Auto-detected via USB enumeration polling after 2s. Manual cycling via FN+Del (`CK_OSMODE`). Once manually set, `os_mode_manual` prevents auto-detection override. RGB indicator: red=none, green=linux, yellow=mac, blue=windows.
+
+OS detection on System76's QMK fork often misidentifies Linux as Windows — both currently map to `OS_MODE_LINUX`.
+
+### Nix Build
+
+The flake copies `keymaps/custom/` into the QMK tree during `postUnpack`. `patchShebangs` is needed for `uf2conv.py` in the sandbox. `python3` and `python3Packages.appdirs` are required nativeBuildInputs.

README.md

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+# System76 Launch 3 Custom Firmware
+
+Custom QMK firmware for the [System76 Launch 3](https://system76.com/accessories/launch) keyboard, built with Nix.
+
+## Features
+
+### OS Mode
+
+The keyboard auto-detects the host OS on plug-in and rewrites shortcuts so that muscle memory from Linux (Ctrl-based shortcuts) works on macOS. Mode can also be cycled manually with **FN+Del**.
+
+Four modes, indicated by RGB flash color:
+
+| Mode    | Color  | Behavior |
+|---------|--------|----------|
+| None    | Red    | No rewrites, all keys pass through |
+| Linux   | Green  | Super+Up → Super+W (KDE overview) |
+| Mac     | Yellow | Full Ctrl→Cmd rewrite (see table below) |
+| Windows | Blue   | Super+Up → Super+Tab (Task View) |
+
+#### Mac Mode Shortcut Rewrites
+
+| Input (Linux muscle memory) | Output (macOS) | Action |
+|---|---|---|
+| Ctrl+C/V | Cmd+C/V | Copy/Paste |
+| Ctrl+Shift+C/V | Cmd+C/V | Terminal copy/paste |
+| Ctrl+X/A/S/F | Cmd+X/A/S/F | Cut, Select All, Save, Find |
+| Ctrl+W/L/R | Cmd+W/L/R | Close tab, Address bar, Reload |
+| Ctrl+Z/T/N | Cmd+Z/T/N | Undo, New tab, New window |
+| Ctrl+Shift+Z/T/N | Cmd+Shift+Z/T/N | Redo, Reopen tab, Incognito |
+| Ctrl+Left/Right | Alt+Left/Right | Word navigation |
+| Ctrl+Alt+Left/Right | Ctrl+Left/Right | Switch workspace |
+| Ctrl+Backspace | Alt+Backspace | Delete word |
+| Home/End | Cmd+Left/Right | Line start/end |
+| Alt+F4 | Cmd+Q | Quit app |
+| Alt+Tab | Cmd+Tab | App switcher |
+| Super tap | Cmd+Space | Spotlight |
+| Super+Up | Ctrl+Up | Mission Control |
+
+#### Layer Override
+
+When any modifier is held (in any mode except None), the FN layer is bypassed — keys always resolve from Layer 0. This allows bypassing shortcut rewrites when wanted.  For example, if you want to send Ctrl+c on mac (which is normally rewritten to cmd+c), simply use fn+Ctrl+c instead.
+
+### Layer 1 (FN Layer)
+
+| Key | FN+ |
+|-----|-----|
+| ESC | Print Screen |
+| F1/F2/F3 | Mute / Vol Down / Vol Up |
+| 1/2/3 | Play-Pause / Prev / Next |
+| 0/-/= | LED Toggle / LED Down / LED Up |
+| Del | Cycle OS Mode |
+| Arrows | Home / PgDn / PgUp / End |
+
+Modifier keys (Shift, Ctrl, GUI, Alt) cannot be rebound on Layer 1 due to the layer override behavior.
+
+## Building
+
+Requires [Nix](https://nixos.org/) with flakes enabled.
+
+```sh
+nix build
+```
+
+The `.uf2` firmware file is output to `./result/`.
+
+### Flashing
+
+1. Put the keyboard into bootloader mode (press the reset button on the bottom)
+2. Copy the `.uf2` file to the mounted USB drive
+
+### Development Shell
+
+```sh
+nix develop
+qmk compile -kb system76/launch_3 -km custom
+```
+
+## Project Structure
+
+```
+keymaps/custom/
+├── keymap.c       # Keymaps, shortcut rewrites, layer override
+├── os_detect.c    # OS auto-detection, RGB flash indicator
+├── os_mode.h      # OS mode enum and shared state
+└── rules.mk       # QMK build flags
+qmk_firmware/      # System76 QMK fork (git submodule)
+flake.nix          # Nix build system
+```