perf(sessions): fold sessions+previews into one batched SSH round-trip

Audit Finding 1 — ChatViewModel.loadRecentSessions and
SessionsViewModel.load each fired two sequential `await
dataService.fetch*` calls (sessions + previews), paying the 420 ms
SSH RTT twice on every reload. Visible in ScarfMon traces as
back-to-back `ssh.run` intervals, totaling ~840 ms minimum
overhead per sidebar refresh.

Adds HermesDataService.sessionListSnapshot(limit:) — same shape
as the existing dashboardSnapshot, folds both queries into a
single backend.queryBatch() call. Both call sites switched.

Halves the SSH round-trips for every sidebar load. With Finding 5's
coalescing, redundant parallel reloads also become free. Together,
the 9× redundant queries-per-minute observed in baseline captures
should drop substantially.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scarf/Packages/ScarfCore/Sources/ScarfCore/Services/HermesDataService.swift

@@ -605,6 +605,60 @@ public actor HermesDataService {
         }
     }
 
+    /// Bundle for the chat sidebar / Sessions tab loaders. Folds
+    /// `fetchSessions(limit:)` + `fetchSessionPreviews(limit:)` into
+    /// one `queryBatch()` round-trip — same shape as
+    /// `dashboardSnapshot`. Pre-fix `ChatViewModel.loadRecentSessions`
+    /// + `SessionsViewModel.load` each fired the two `await
+    /// dataService.fetch*` calls in serial, paying the SSH RTT
+    /// twice (~840 ms minimum on a 420 ms-RTT remote, observed in
+    /// ScarfMon `mac.loadRecentSessions` traces). Halves the
+    /// round-trips for every sidebar load. Each tick still pays
+    /// for `dashboard.loadRegistry` separately because that's a
+    /// projects.json read (not SQL) and goes through a different
+    /// transport call.
+    public struct SessionListSnapshot: Sendable {
+        public let sessions: [HermesSession]
+        public let previews: [String: String]
+    }
+
+    public func sessionListSnapshot(limit: Int = QueryDefaults.sessionLimit) async -> SessionListSnapshot {
+        let previewLimit = limit
+        let statements: [(sql: String, params: [SQLValue])] = [
+            (
+                "SELECT \(sessionColumns) FROM sessions WHERE parent_session_id IS NULL ORDER BY started_at DESC LIMIT ?",
+                [.integer(Int64(limit))]
+            ),
+            (
+                """
+                SELECT m.session_id, substr(m.content, 1, \(QueryDefaults.previewContentLength))
+                FROM messages m
+                INNER JOIN (
+                    SELECT session_id, MIN(id) as min_id
+                    FROM messages
+                    WHERE role = 'user' AND content <> ''
+                    GROUP BY session_id
+                ) first ON m.id = first.min_id
+                ORDER BY m.timestamp DESC
+                LIMIT ?
+                """,
+                [.integer(Int64(previewLimit))]
+            )
+        ]
+        do {
+            let resultSets = try await backend.queryBatch(statements)
+            let sessions = (resultSets.first ?? []).map { sessionFromRow($0) }
+            var previews: [String: String] = [:]
+            for row in (resultSets.count > 1 ? resultSets[1] : []) {
+                previews[row.string(at: 0)] = row.string(at: 1)
+            }
+            return SessionListSnapshot(sessions: sessions, previews: previews)
+        } catch {
+            Self.logger.warning("sessionListSnapshot failed: \(error.localizedDescription, privacy: .public)")
+            return SessionListSnapshot(sessions: [], previews: [:])
+        }
+    }
+
     /// Bundle the queries Insights fires on every load into one
     /// backend round-trip — same rationale as `dashboardSnapshot`.
     public struct InsightsSnapshot: Sendable {

scarf/scarf/Features/Chat/ViewModels/ChatViewModel.swift

@@ -947,8 +947,16 @@ final class ChatViewModel {
             // getting truncated out of the original limit). Sessions feature
             // loads 500; the chat sidebar doesn't need that, but 50 keeps
             // the project filter useful without measurable cost.
-            let fetchedSessions = await dataService.fetchSessions(limit: 50)
-            let fetchedPreviews = await dataService.fetchSessionPreviews(limit: 50)
+            //
+            // v2.7: folded sessions + previews into one queryBatch round
+            // trip via sessionListSnapshot. Pre-fix the two awaits below
+            // were serialized SSH calls, paying the 420 ms RTT twice
+            // every time the file watcher fired (~2.2 s baseline reload).
+            // sessionListSnapshot halves the round-trips for every
+            // sidebar refresh.
+            let snapshot = await dataService.sessionListSnapshot(limit: 50)
+            let fetchedSessions = snapshot.sessions
+            let fetchedPreviews = snapshot.previews
             await dataService.close()
 
             // Project attribution + registry — single batched off-main read.

scarf/scarf/Features/Sessions/ViewModels/SessionsViewModel.swift

@@ -77,8 +77,13 @@ final class SessionsViewModel {
         // re-opening — cleanup() closes on disappear.
         let opened = await dataService.refresh()
         guard opened else { return }
-        sessions = await dataService.fetchSessions(limit: 500)
-        sessionPreviews = await dataService.fetchSessionPreviews(limit: 500)
+        // v2.7: folded the two serial fetches into one batched round
+        // trip via sessionListSnapshot. Pre-fix this paid the 420 ms
+        // SSH RTT twice on every Sessions tab open (~840 ms minimum
+        // for the two queries alone over remote).
+        let snapshot = await dataService.sessionListSnapshot(limit: 500)
+        sessions = snapshot.sessions
+        sessionPreviews = snapshot.previews
 
         // Load attribution + registry off the main actor in one batch so
         // 500 rows don't trigger 500 SFTP reads. Failure is silent — the