Add board presets, devices --clear, and test/UX fixes

Board presets:
- anvil new --board mega (uno, mega, nano, nano-old, leonardo, micro)
- anvil new --list-boards shows presets with compatible clones
- FQBN and baud rate flow into .anvil.toml via template variables
- Defaults to uno when --board is omitted

Devices --clear:
- anvil devices --clear deletes .anvil.local, reverts to auto-detect

src/board/mod.rs

@@ -4,6 +4,8 @@ use serde::Deserialize;
 use std::path::{Path, PathBuf};
 use std::process::Command;
 
+pub mod presets;
+
 /// Information about a detected serial port.
 #[derive(Debug, Clone)]
 pub struct PortInfo {