feat(engine): Phase 6 — bridge adapters for main crate integration

Strategy C parallel deployment: when ENGINE_V2=true env var is set,
user messages route through the engine instead of the existing agentic
loop. All existing behavior is unchanged when the flag is off.

Bridge module (src/bridge/):
- LlmBridgeAdapter: wraps LlmProvider as engine LlmBackend, converts
  ThreadMessage↔ChatMessage, ActionDef↔ToolDefinition, depth-based
  model routing (primary vs cheap_llm)
- EffectBridgeAdapter: wraps ToolRegistry+SafetyLayer as EffectExecutor,
  routes tool calls through existing execute_tool_with_safety pipeline
- InMemoryStore: HashMap-backed Store impl (no DB tables needed yet)
- EngineRouter: is_engine_v2_enabled() + handle_with_engine() that
  builds engine from Agent deps and processes messages end-to-end

Integration touchpoint (4 lines in agent_loop.rs):
  After hook processing, before session resolution, check ENGINE_V2
  flag and route UserInput through the engine path.

Accessor visibility widened: llm(), cheap_llm(), safety(), tools()
changed from pub(super) to pub(crate) for bridge access.

85 engine tests + main crate clippy clean.

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

src/agent/agent_loop.rs

@@ -283,20 +283,20 @@ impl Agent {
         self.deps.store.as_ref()
     }
 
-    pub(super) fn llm(&self) -> &Arc<dyn LlmProvider> {
+    pub(crate) fn llm(&self) -> &Arc<dyn LlmProvider> {
         &self.deps.llm
     }
 
     /// Get the cheap/fast LLM provider, falling back to the main one.
-    pub(super) fn cheap_llm(&self) -> &Arc<dyn LlmProvider> {
+    pub(crate) fn cheap_llm(&self) -> &Arc<dyn LlmProvider> {
         self.deps.cheap_llm.as_ref().unwrap_or(&self.deps.llm)
     }
 
-    pub(super) fn safety(&self) -> &Arc<SafetyLayer> {
+    pub(crate) fn safety(&self) -> &Arc<SafetyLayer> {
         &self.deps.safety
     }
 
-    pub(super) fn tools(&self) -> &Arc<ToolRegistry> {
+    pub(crate) fn tools(&self) -> &Arc<ToolRegistry> {
         &self.deps.tools
     }
 
@@ -1003,6 +1003,13 @@ impl Agent {
             }
         }
 
+        // Engine V2 routing (Strategy C: parallel deployment)
+        if let Submission::UserInput { ref content } = submission
+            && crate::bridge::is_engine_v2_enabled()
+        {
+            return crate::bridge::handle_with_engine(self, message, content).await;
+        }
+
         // Hydrate thread from DB if it's a historical thread not in memory
         if let Some(external_thread_id) = message.conversation_scope() {
             tracing::trace!(