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/tools/hardware_board_info.rs

@@ -124,10 +124,11 @@ impl Tool for HardwareBoardInfoTool {
                     });
                 }
                 Err(e) => {
-                    output.push_str(&format!(
-                        "probe-rs attach failed: {}. Using static info.\n\n",
-                        e
-                    ));
+                    use std::fmt::Write;
+                    let _ = write!(
+                        output,
+                        "probe-rs attach failed: {e}. Using static info.\n\n"
+                    );
                 }
             }
         }
@@ -135,13 +136,15 @@ impl Tool for HardwareBoardInfoTool {
         if let Some(info) = self.static_info_for_board(board) {
             output.push_str(&info);
             if let Some(mem) = memory_map_static(board) {
-                output.push_str(&format!("\n\n**Memory map:**\n{}", mem));
+                use std::fmt::Write;
+                let _ = write!(output, "\n\n**Memory map:**\n{mem}");
             }
         } else {
-            output.push_str(&format!(
-                "Board '{}' configured. No static info available.",
-                board
-            ));
+            use std::fmt::Write;
+            let _ = write!(
+                output,
+                "Board '{board}' configured. No static info available."
+            );
         }
 
         Ok(ToolResult {