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/commands/refresh.rs

@@ -47,6 +47,8 @@ pub fn run_refresh(project_dir: Option<&str>, force: bool) -> Result<()> {
     let context = TemplateContext {
         project_name: config.project.name.clone(),
         anvil_version: ANVIL_VERSION.to_string(),
+        fqbn: config.build.fqbn.clone(),
+        baud: config.monitor.baud,
     };
 
     // Extract template into a temp directory so we can compare