fix: resolve all clippy warnings, formatting, and Mistral endpoint

- Fix Mistral provider base URL (missing /v1 prefix caused 404s)
- Resolve 55 clippy warnings across 28 warning types
- Apply cargo fmt to 44 formatting violations
- Remove unused imports (process_message, MultiObserver, VerboseObserver,
  ChatResponse, ToolCall, Path, TempDir)
- Replace format!+push_str with write! macro
- Fix unchecked Duration subtraction, redundant closures, clamp patterns
- Declare missing feature flags (sandbox-landlock, sandbox-bubblewrap,
  browser-native) in Cargo.toml
- Derive Default where manual impls were redundant
- Add separators to long numeric literals (115200 → 115_200)
- Restructure unreachable code in arduino_flash platform branches

All 1,500 tests pass. Zero clippy warnings. Clean formatting.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/peripherals/arduino_flash.rs

@@ -38,6 +38,10 @@ pub fn ensure_arduino_cli() -> Result<()> {
             anyhow::bail!("brew install arduino-cli failed. Install manually: https://arduino.github.io/arduino-cli/");
         }
         println!("arduino-cli installed.");
+        if !arduino_cli_available() {
+            anyhow::bail!("arduino-cli still not found after install. Ensure it's in PATH.");
+        }
+        return Ok(());
     }
 
     #[cfg(target_os = "linux")]
@@ -54,11 +58,6 @@ pub fn ensure_arduino_cli() -> Result<()> {
         println!("arduino-cli not found. Install it: https://arduino.github.io/arduino-cli/");
         anyhow::bail!("arduino-cli not installed.");
     }
-
-    if !arduino_cli_available() {
-        anyhow::bail!("arduino-cli still not found after install. Ensure it's in PATH.");
-    }
-    Ok(())
 }
 
 /// Ensure arduino:avr core is installed.

src/peripherals/serial.rs

@@ -112,6 +112,7 @@ pub struct SerialPeripheral {
 
 impl SerialPeripheral {
     /// Create and connect to a serial peripheral.
+    #[allow(clippy::unused_async)]
     pub async fn connect(config: &PeripheralBoardConfig) -> anyhow::Result<Self> {
         let path = config
             .path