feat(adr): implement ADR-0109, ADR-0110, and ADR-0111

- ADR-0109: Incremental Background AST Indexing & Differential Graph Updates via Tree-Sitter
- ADR-0110: Real-Time SSE Live Activity & Log Stream (/api/events/stream)
- ADR-0111: Automated Stale Symbol Pruning & Graph Tombstoning (sweep_graph_health auto_prune_stale_files)

server/src/api/events.rs

@@ -2,8 +2,11 @@ use crate::AppState;
 use crate::state::GenericEvent;
 use axum::extract::{Query, State};
 use axum::response::IntoResponse;
+use axum::response::sse::{Event, KeepAlive, Sse};
 use std::collections::HashMap;
 use std::sync::Arc;
+use tokio_stream::StreamExt;
+use tokio_stream::wrappers::BroadcastStream;
 
 pub async fn wait_for_event_handler(
     State(state): State<Arc<AppState>>,
@@ -49,6 +52,36 @@ pub async fn post_event_handler(
     axum::Json(serde_json::json!({"status": "ok"}))
 }
 
+/// Real-time Server-Sent Events (SSE) stream for agent execution events and observability (ADR-0110)
+pub async fn sse_events_handler(
+    State(state): State<Arc<AppState>>,
+    Query(params): Query<HashMap<String, String>>,
+) -> impl IntoResponse {
+    let topic_filter = params.get("topic").cloned();
+    let session_filter = params.get("session_id").cloned();
+    let rx = state.handler.state.event_bus_tx.subscribe();
+
+    let stream = BroadcastStream::new(rx).filter_map(move |msg| match msg {
+        Ok(event) => {
+            let topic_matches = topic_filter.as_ref().is_none_or(|t| t == &event.topic);
+            let session_matches = session_filter
+                .as_ref()
+                .is_none_or(|s| Some(s) == event.session_id.as_ref());
+            if topic_matches && session_matches {
+                let json_data = serde_json::to_string(&event).unwrap_or_default();
+                Some(Ok::<_, std::convert::Infallible>(
+                    Event::default().event(event.topic).data(json_data),
+                ))
+            } else {
+                None
+            }
+        }
+        Err(_) => None,
+    });
+
+    Sse::new(stream).keep_alive(KeepAlive::default())
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;