fix: avoid repeated activity history reads during streaming

Reuse the last committed activity records when comparing streaming saves instead of reading the full history for every update. Invalidate comparisons on close, data clearing and commits from other SQLite connections; preserve rollback and retry behavior.

Validation: 3 related test files / 121 tests passed; typecheck and lint passed. Ten consecutive saves now perform at most one full-history scan while preserving the WAL write reduction. Installed-app physical disk throughput remains unverified.

Release note: Reduce repeated database reads while streaming activity updates in conversations with large activity histories.

src/main/assistant/assistant-database.ts

@@ -1799,6 +1799,7 @@ export class AssistantDatabase {
   private static readonly executionStatsCacheTtlMs = 30_000
   private static readonly executionStatsCacheLimit = 8
   private database?: DatabaseSync
+  private activityHistoryCache?: { records: Map<string, string>; dataVersion: number }
   private executionStatsReader?: ExecutionStatsReader
   private readonly noteStorage: MagicNoteStorage
   private readonly dirtyMagicNotes = new Set<string>()
@@ -2220,6 +2221,7 @@ export class AssistantDatabase {
     this.executionStatsReader?.close()
     this.executionStatsReader = undefined
     this.executionStatsCache.clear()
+    this.activityHistoryCache = undefined
     this.database?.close()
     this.database = undefined
     this.subagentProgress = undefined
@@ -2277,6 +2279,7 @@ export class AssistantDatabase {
       throw error
     }
     this.dirtyMagicNotes.clear()
+    this.activityHistoryCache = undefined
     this.pendingMagicNoteCleanup.clear()
     this.reconcileMagicNoteFiles()
     this.options.onMagicNotesChanged?.()
@@ -4877,8 +4880,13 @@ export class AssistantDatabase {
     const database = this.requireDatabase()
     database.exec('BEGIN IMMEDIATE')
     try {
-      this.writeActivityHistory(database, snapshot)
+      const { data_version: dataVersion } = database.prepare('PRAGMA data_version').get() as { data_version: number }
+      const previous = this.activityHistoryCache?.dataVersion === dataVersion
+        ? this.activityHistoryCache.records : undefined
+      const records = this.writeActivityHistory(database, snapshot, undefined, previous)
       database.exec('COMMIT')
+      // Publish only committed records; a failed save must remain retryable.
+      this.activityHistoryCache = { records, dataVersion }
     } catch (error) {
       database.exec('ROLLBACK')
       throw error
@@ -4888,12 +4896,16 @@ export class AssistantDatabase {
   private writeActivityHistory(
     database: DatabaseSync,
     snapshot: ActivityHistorySnapshot,
-    onProgress?: (processed: number) => void
-  ): void {
-    const rows = database.prepare(
-      'SELECT record_key, record_json FROM activity_history_records'
-    ).all() as Array<{ record_key: string; record_json: string }>
-    const previous = new Map(rows.map((row) => [row.record_key, row.record_json]))
+    onProgress?: (processed: number) => void,
+    previous?: ReadonlyMap<string, string>
+  ): Map<string, string> {
+    if (!previous) {
+      const rows = database.prepare(
+        'SELECT record_key, record_json FROM activity_history_records'
+      ).all() as Array<{ record_key: string; record_json: string }>
+      previous = new Map(rows.map((row) => [row.record_key, row.record_json]))
+    }
+    const records = new Map<string, string>()
     const occurrences = new Map<string, number>()
     const order: string[] = []
     const upsert = database.prepare(
@@ -4909,10 +4921,12 @@ export class AssistantDatabase {
       const json = JSON.stringify(record)
       order.push(key)
       if (previous.get(key) !== json) upsert.run(key, json)
-      previous.delete(key)
+      records.set(key, json)
     }
     const remove = database.prepare('DELETE FROM activity_history_records WHERE record_key = ?')
-    for (const key of previous.keys()) remove.run(key)
+    for (const key of previous.keys()) {
+      if (!records.has(key)) remove.run(key)
+    }
     const orderJson = JSON.stringify(order)
     const incomplete = Number(snapshot.legacyHistoryMayBeIncomplete)
     database.prepare(
@@ -4921,6 +4935,7 @@ export class AssistantDatabase {
        WHERE singleton = 1
          AND (record_order_json != ? OR legacy_history_may_be_incomplete != ?)`
     ).run(orderJson, incomplete, orderJson, incomplete)
+    return records
   }
 
   createTask(input: {