`src/middleware/shield.rs` — `SafetyShieldMiddleware` [feature: tool_shield]

The pipeline wiring for tool safety shields.

Key items

• SafetyShieldMiddleware::new(shield: Arc<dyn ToolSafetyShield>) — name "safety_shield"; maintains the last-20 watched-calls window that becomes ShieldContext::recent_calls.

Behavior notes

• Block → soft error "blocked by safety shield: <reason>", zero duration — run continues. Warn → tracing warning, call proceeds. Allow → proceed.
• Every admitted watched call is recorded via shield.record_invocation (blocked attempts never are; an inner cache-served repeat still is).
• Install order: after renaming middleware (evaluate the executed name), before memoize (score repetitions even on cache hits), exactly once.
• Repeated blocks count as failures toward the tool’s breaker — once open, the engine’s gate refuses first and the model sees “temporarily unavailable”.
• An empty watched set skips the middleware entirely.

Deep dive: The shield.