Update flake.nix and README.md for Raspberry Pi Pico W support

flake.nix

@@ -73,16 +73,11 @@
               rustup
               # Embedded Rust tooling
               probe-rs-tools
-              espflash
               openocd
               minicom
               # RP2040 UF2 workflows
               elf2uf2-rs
               picotool
-              avrdude
-              pkgsCross.avr.buildPackages.libc
-              pkgsCross.avr.buildPackages.gcc
-              ravedude
               pre-commit
             ];
 
@@ -102,15 +97,11 @@
               export PATH="$HOME/.cargo/bin:$PATH"
 
               if command -v rustup >/dev/null 2>&1; then
-                echo "Ensuring common embedded Rust targets are available (idempotent)..."
+                echo "Ensuring Raspberry Pi Pico W target is available (idempotent)..."
                 rustup target add thumbv6m-none-eabi       >/dev/null 2>&1 || true  # Cortex-M0/M0+ (RP2040)
-                rustup target add thumbv7em-none-eabihf    >/dev/null 2>&1 || true  # Cortex-M4F/M7F (many STM32/nRF52)
-                rustup target add riscv32imc-unknown-none-elf >/dev/null 2>&1 || true
-                rustup target add riscv32imac-unknown-none-elf >/dev/null 2>&1 || true  # ESP32-C3 class
-                echo "Note: ESP32 Xtensa targets require espup/custom toolchains (see esp-rs)."
               fi
               echo "[rust-template] Welcome to the Rust dev shell!"
-              echo "Tools: cargo, rustc, clippy, rustfmt, rust-analyzer, probe-rs, espflash, openocd, minicom, elf2uf2-rs, picotool, avrdude, ravedude"
+              echo "Tools: cargo, rustc, clippy, rustfmt, rust-analyzer, probe-rs, openocd, minicom, elf2uf2-rs, picotool"
               echo "Run 'cargo build' to build, or 'nix run .#dev-helper' for a tool summary."
               echo "See README.md for usage."
             '';