feat(bridge): integrate with web gateway via AppEvent + v1 conversation DB

Three changes to make engine v2 visible in the web gateway:

1. SSE event streaming (AppEvent broadcast):
   - ThreadEvent → AppEvent conversion via thread_event_to_app_event()
   - Events broadcast to SseManager during the poll loop
   - Covers: Thinking, ToolCompleted (success/error), Status, Response
   - Web gateway receives real-time progress without any gateway changes

2. Conversation persistence to v1 database:
   - After thread completes, writes user message + agent response to
     v1 ConversationStore via add_conversation_message()
   - Uses get_or_create_assistant_conversation() for per-user per-channel
   - Web gateway reads from DB as usual — chat history appears

3. Final response broadcast:
   - AppEvent::Response with full text + thread_id sent via SSE
   - Web gateway renders the response in the chat UI

New EngineState fields: sse (Option<Arc<SseManager>>),
db (Option<Arc<dyn Database>>). Both populated from Agent.deps.

Agent.deps visibility widened to pub(crate).

Depends on: ironclaw_common crate with AppEvent type (PR #1615).

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

src/agent/agent_loop.rs

@@ -177,7 +177,7 @@ pub struct AgentDeps {
 /// The main agent that coordinates all components.
 pub struct Agent {
     pub(super) config: AgentConfig,
-    pub(super) deps: AgentDeps,
+    pub(crate) deps: AgentDeps,
     pub(crate) channels: Arc<ChannelManager>,
     pub(super) context_manager: Arc<ContextManager>,
     pub(super) scheduler: Arc<Scheduler>,