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/memory/response_cache.rs

@@ -166,7 +166,7 @@ impl ResponseCache {
             |row| row.get(0),
         )?;
 
-        #[allow(clippy::cast_sign_loss)]
+        #[allow(clippy::cast_sign_loss, clippy::cast_possible_truncation)]
         Ok((count as usize, hits as u64, tokens_saved as u64))
     }