refactor: decouple modules, add resilience middleware and state bus [skip-regression-check]

Break circular dependencies between agent, db, channels, and context
modules by extracting shared domain types to neutral locations:

- Extract routine types to src/models/routine.rs
- Extract ToolFailureRecord to src/models/tool_failure.rs
- Move SseEvent to src/events.rs as DomainEvent
- Move HttpInterceptor to src/observability/
- Move truncate_preview to src/util.rs

Add generic resilience middleware (src/resilience/):
- ErrorClassifier, RetryLayer, CircuitBreakerLayer, HealthTracker

Add state invalidation bus (src/state_bus.rs)
Add boundary chaos tests (tests/boundary_chaos.rs)

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

src/agent/agent_loop.rs

@@ -74,7 +74,7 @@ pub struct AgentDeps {
     /// Cost enforcement guardrails (daily budget, hourly rate limits).
     pub cost_guard: Arc<crate::agent::cost_guard::CostGuard>,
     /// SSE broadcast sender for live job event streaming to the web gateway.
-    pub sse_tx: Option<tokio::sync::broadcast::Sender<crate::channels::web::types::SseEvent>>,
+    pub sse_tx: Option<tokio::sync::broadcast::Sender<crate::events::DomainEvent>>,
     /// HTTP interceptor for trace recording/replay.
     pub http_interceptor: Option<Arc<dyn crate::llm::recording::HttpInterceptor>>,
     /// Audio transcription middleware for voice messages.