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/devices.rs

@@ -274,6 +274,31 @@ pub fn set_port(port: Option<&str>, project_dir: Option<&str>) -> Result<()> {
 
 // -- Helpers --------------------------------------------------------------
 
+/// Delete .anvil.local from the given project directory.
+pub fn clear_port(project_dir: Option<&str>) -> Result<()> {
+    let project_path = resolve_project_dir(project_dir)?;
+    require_anvil_project(&project_path)?;
+
+    let local_file = project_path.join(".anvil.local");
+    if !local_file.exists() {
+        println!(
+            "{} No .anvil.local file found -- nothing to clear.",
+            "--".bright_black()
+        );
+        return Ok(());
+    }
+
+    fs::remove_file(&local_file)
+        .context("Failed to delete .anvil.local")?;
+
+    println!(
+        "{} Removed .anvil.local -- port will be auto-detected.",
+        "ok".green()
+    );
+
+    Ok(())
+}
+
 fn resolve_project_dir(project_dir: Option<&str>) -> Result<PathBuf> {
     match project_dir {
         Some(dir) => Ok(PathBuf::from(dir)),