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/llm/recording.rs

@@ -51,32 +51,10 @@ pub struct MemorySnapshotEntry {
     pub content: String,
 }
 
-/// A recorded HTTP request/response pair.
-#[derive(Debug, Clone, Serialize, Deserialize)]
-pub struct HttpExchange {
-    pub request: HttpExchangeRequest,
-    pub response: HttpExchangeResponse,
-}
-
-/// The request side of an HTTP exchange.
-#[derive(Debug, Clone, Serialize, Deserialize)]
-pub struct HttpExchangeRequest {
-    pub method: String,
-    pub url: String,
-    #[serde(default, skip_serializing_if = "Vec::is_empty")]
-    pub headers: Vec<(String, String)>,
-    #[serde(default, skip_serializing_if = "Option::is_none")]
-    pub body: Option<String>,
-}
-
-/// The response side of an HTTP exchange.
-#[derive(Debug, Clone, Serialize, Deserialize)]
-pub struct HttpExchangeResponse {
-    pub status: u16,
-    #[serde(default, skip_serializing_if = "Vec::is_empty")]
-    pub headers: Vec<(String, String)>,
-    pub body: String,
-}
+// Re-export HTTP exchange types from their canonical location in observability.
+pub use crate::observability::http_interceptor::{
+    HttpExchange, HttpExchangeRequest, HttpExchangeResponse, HttpInterceptor,
+};
 
 /// A single step in the trace.
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -144,23 +122,7 @@ pub struct ExpectedToolResult {
     pub content: String,
 }
 
-// ── HTTP interceptor ───────────────────────────────────────────────
-
-/// Trait for intercepting HTTP requests from tools.
-///
-/// During recording, the interceptor captures exchanges after the real
-/// request completes. During replay, it short-circuits with a recorded response.
-#[async_trait]
-pub trait HttpInterceptor: Send + Sync + std::fmt::Debug {
-    /// Called before making an HTTP request.
-    ///
-    /// Return `Some(response)` to short-circuit (replay mode).
-    /// Return `None` to let the real request proceed (recording mode).
-    async fn before_request(&self, request: &HttpExchangeRequest) -> Option<HttpExchangeResponse>;
-
-    /// Called after a real HTTP request completes (recording mode only).
-    async fn after_response(&self, request: &HttpExchangeRequest, response: &HttpExchangeResponse);
-}
+// ── HTTP interceptor impls ─────────────────────────────────────────
 
 /// Records HTTP exchanges during a live session.
 #[derive(Debug)]