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/templates/mod.rs

@@ -10,6 +10,8 @@ static BASIC_TEMPLATE: Dir = include_dir!("$CARGO_MANIFEST_DIR/templates/basic")
 pub struct TemplateContext {
     pub project_name: String,
     pub anvil_version: String,
+    pub fqbn: String,
+    pub baud: u32,
 }
 
 pub struct TemplateManager;
@@ -140,6 +142,8 @@ fn substitute_variables(text: &str, context: &TemplateContext) -> String {
     text.replace("{{PROJECT_NAME}}", &context.project_name)
         .replace("{{ANVIL_VERSION}}", &context.anvil_version)
         .replace("{{ANVIL_VERSION_CURRENT}}", ANVIL_VERSION)
+        .replace("{{FQBN}}", &context.fqbn)
+        .replace("{{BAUD}}", &context.baud.to_string())
 }
 
 #[cfg(test)]
@@ -172,10 +176,15 @@ mod tests {
         let ctx = TemplateContext {
             project_name: "my_project".to_string(),
             anvil_version: "1.0.0".to_string(),
+            fqbn: "arduino:avr:mega:cpu=atmega2560".to_string(),
+            baud: 9600,
         };
-        let input = "Name: {{PROJECT_NAME}}, Version: {{ANVIL_VERSION}}";
+        let input = "Name: {{PROJECT_NAME}}, Version: {{ANVIL_VERSION}}, FQBN: {{FQBN}}, Baud: {{BAUD}}";
         let output = substitute_variables(input, &ctx);
-        assert_eq!(output, "Name: my_project, Version: 1.0.0");
+        assert_eq!(
+            output,
+            "Name: my_project, Version: 1.0.0, FQBN: arduino:avr:mega:cpu=atmega2560, Baud: 9600"
+        );
     }
 
     #[test]
@@ -190,6 +199,8 @@ mod tests {
         let ctx = TemplateContext {
             project_name: "test_proj".to_string(),
             anvil_version: "1.0.0".to_string(),
+            fqbn: "arduino:avr:uno".to_string(),
+            baud: 115200,
         };
 
         let count = TemplateManager::extract("basic", tmp.path(), &ctx).unwrap();
@@ -220,6 +231,8 @@ mod tests {
         let ctx = TemplateContext {
             project_name: "my_sensor".to_string(),
             anvil_version: "1.0.0".to_string(),
+            fqbn: "arduino:avr:uno".to_string(),
+            baud: 115200,
         };
 
         TemplateManager::extract("basic", tmp.path(), &ctx).unwrap();
@@ -231,4 +244,4 @@ mod tests {
             ".anvil.toml should contain project name"
         );
     }
-}
+}