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/agent/mod.rs

@@ -7,7 +7,7 @@ pub mod prompt;
 
 #[allow(unused_imports)]
 pub use agent::{Agent, AgentBuilder};
-pub use loop_::{process_message, run};
+pub use loop_::run;
 
 #[cfg(test)]
 mod tests {
@@ -18,7 +18,6 @@ mod tests {
     #[test]
     fn run_function_is_reexported() {
         assert_reexport_exists(run);
-        assert_reexport_exists(process_message);
         assert_reexport_exists(loop_::run);
         assert_reexport_exists(loop_::process_message);
     }