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);