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/observability/mod.rs

@@ -6,11 +6,9 @@ pub mod traits;
 pub mod verbose;
 
 pub use self::log::LogObserver;
-pub use self::multi::MultiObserver;
 pub use noop::NoopObserver;
 pub use otel::OtelObserver;
 pub use traits::{Observer, ObserverEvent};
-pub use verbose::VerboseObserver;
 
 use crate::config::ObservabilityConfig;