perf(dashboard): kill startup SSE storm and slim hot-path task scans

- TaskStore.watch() now initializes lastPollTime so the first
  checkForChanges() poll filters by "modified since now" instead of
  doing an unfiltered SELECT * and emitting a task:updated event for
  every cached task. On a 1200-task board this dropped ~60 MB of SSE
  traffic and a 1199-call setState storm one second after dashboard
  startup.
- listTasks() gains a column option so callers can filter in SQL.
- dashboard CLI auto-merge sweeps (startup + 2 unpause handlers + the
  15s periodic retry) now use listTasks({ column: "in-review" })
  instead of pulling the full table on every cycle.
- self-healing archiveStaleDoneTasks() uses slim listTasks — it only
  needs id/column/columnMovedAt to decide staleness.
- Document the listTasks() perf contract and the watch() polling
  invariant in AGENTS.md and project memory.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/cli/src/commands/dashboard.ts

@@ -876,7 +876,7 @@ export async function runDashboard(port: number, opts: { paused?: boolean; dev?:
 
     // ── Startup sweep: enqueue any tasks already in "in-review" ───────
     if (settings.autoMerge) {
-      const existing = await store.listTasks();
+      const existing = await store.listTasks({ column: "in-review" });
       const inReview = existing.filter((t) => canAutoMergeTask(t as any));
       if (inReview.length > 0) {
         console.log(
@@ -911,7 +911,7 @@ export async function runDashboard(port: number, opts: { paused?: boolean; dev?:
 
         if (s.autoMerge) {
           try {
-            const tasks = await store.listTasks();
+            const tasks = await store.listTasks({ column: "in-review" });
             for (const t of tasks) {
               if (canAutoMergeTask(t as any)) {
                 enqueueMerge(t.id);
@@ -935,7 +935,7 @@ export async function runDashboard(port: number, opts: { paused?: boolean; dev?:
 
         if (s.autoMerge) {
           try {
-            const tasks = await store.listTasks();
+            const tasks = await store.listTasks({ column: "in-review" });
             for (const t of tasks) {
               if (canAutoMergeTask(t as any)) {
                 enqueueMerge(t.id);
@@ -999,7 +999,7 @@ export async function runDashboard(port: number, opts: { paused?: boolean; dev?:
           // Refresh the cached limit so the semaphore picks up live changes
           cachedMaxConcurrent = s.maxConcurrent;
           if (!s.globalPause && !s.enginePaused && s.autoMerge) {
-            const tasks = await store.listTasks();
+            const tasks = await store.listTasks({ column: "in-review" });
             for (const t of tasks) {
               if (canAutoMergeTask(t as any)) {
                 enqueueMerge(t.id);

packages/core/src/store.ts

@@ -1316,9 +1316,12 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
      *  from each row to make list responses cheap for board-style consumers. Detail fields default
      *  to empty arrays in the returned Task objects; use `getTask(id)` to load full data. */
     slim?: boolean;
+    /** Restrict to a single column (e.g. 'in-review' for the auto-merge sweep). */
+    column?: Column;
   }): Promise<Task[]> {
     const includeArchived = options?.includeArchived ?? true;
     const slim = options?.slim ?? false;
+    const columnFilter = options?.column;
 
     const slimColumns = `
       id, title, description, "column", status, size, reviewLevel, currentStep,
@@ -1336,10 +1339,18 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
       checkedOutBy, checkedOutAt
     `;
     const selectClause = slim ? slimColumns : '*';
-    const whereClause = includeArchived ? '' : ` WHERE "column" != 'archived'`;
+    const whereParts: string[] = [];
+    const params: string[] = [];
+    if (columnFilter) {
+      whereParts.push(`"column" = ?`);
+      params.push(columnFilter);
+    } else if (!includeArchived) {
+      whereParts.push(`"column" != 'archived'`);
+    }
+    const whereClause = whereParts.length > 0 ? ` WHERE ${whereParts.join(" AND ")}` : "";
     const sql = `SELECT ${selectClause} FROM tasks${whereClause} ORDER BY createdAt ASC`;
 
-    const rows = this.db.prepare(sql).all();
+    const rows = this.db.prepare(sql).all(...params);
     const tasks = (rows as any[]).map((row) => this.rowToTask(row));
 
     // Sort by createdAt, then by numeric ID suffix for tie-breaking
@@ -2699,6 +2710,11 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
 
     // Store current lastModified
     this.lastKnownModified = this.db.getLastModified();
+    // Initialize lastPollTime so the first checkForChanges() cycle filters by
+    // "modified since now" instead of doing a full SELECT * + emitting an
+    // update event for every cached task. Without this, dashboard startup
+    // re-loaded the entire tasks table 1s after watch() began.
+    this.lastPollTime = new Date().toISOString();
 
     // Use a sentinel watcher object so existing code that checks `this.watcher` still works
     try {

packages/engine/src/self-healing.ts

@@ -365,7 +365,10 @@ export class SelfHealingManager {
 
   async archiveStaleDoneTasks(): Promise<number> {
     try {
-      const tasks = await this.store.listTasks();
+      // Slim listing — we only need id/column/columnMovedAt/updatedAt to decide
+      // staleness. Pulling full task payloads (logs, comments, steps) here used
+      // to drag in tens of MB on busy boards and stalled the maintenance loop.
+      const tasks = await this.store.listTasks({ slim: true });
       const cutoff = Date.now() - SelfHealingManager.AUTO_ARCHIVE_AFTER_MS;
 
       const stale = tasks.filter((t) => {