perf(server): read the subscription freshness head without fetching the transcript

The freshness check read the head by fetching every row of the agent's history on each subscribe. The timeline head now carries its last committed sequence number, so the check reads only the head.

packages/server/src/server/agent/agent-manager.ts

@@ -85,6 +85,7 @@ import {
   AgentTimeline,
   type TimelineAppendResult,
   type TimelineCommit,
+  type TimelineHead,
   type TimelineEntry,
   type TimelineIngestResult,
   type TimelineRetirementCommit,
@@ -1449,20 +1450,10 @@ export class AgentManager {
     return await this.timeline.readRows({ agentId: id });
   }
 
-  /**
-   * The timeline head of a loaded agent, or null when this agent is not loaded. Never loads.
-   * `seq` is the last committed sequence number, a retirement's included.
-   */
-  async getLoadedTimelineHead(
-    id: string,
-  ): Promise<{ epoch: string; seq: number; retirementCount: number } | null> {
+  /** The timeline head of a loaded agent, or null when this agent is not loaded. Never loads. */
+  async getLoadedTimelineHead(id: string): Promise<TimelineHead | null> {
     if (!this.getAgent(id)) return null;
-    const page = await this.timeline.fetch({ agentId: id }, { direction: "tail", limit: 0 });
-    return {
-      epoch: page.epoch,
-      seq: page.window.nextSeq - 1,
-      retirementCount: page.retirementCount,
-    };
+    return await this.timeline.describe({ agentId: id });
   }
 
   async fetchTimeline(

packages/server/src/server/agent/timeline/agent-timeline.test.ts

@@ -208,6 +208,7 @@ describe.each(
     expect(await timeline.describe(child)).toBeNull();
     expect(await timeline.describe(root("a"))).toEqual({
       epoch: "parent",
+      seq: 1,
       size: 1,
       retirementCount: 0,
       rewind: null,
@@ -765,6 +766,7 @@ describe.each(TIMELINE_CONTRACT_BACKENDS)(
       expect(texts(backend.retiredRows(scope))).toEqual(["b", "tool_call", "B"]);
       expect(await timeline.describe(scope)).toEqual({
         epoch: "e",
+        seq: 8,
         size: 4,
         retirementCount: 1,
         rewind: null,

packages/server/src/server/agent/timeline/internal/backend.ts

@@ -32,6 +32,8 @@ export interface TimelineSeed {
 
 export interface TimelineHead {
   epoch: string;
+  /** The last sequence number committed, a retirement's included; 0 before any commit. */
+  seq: number;
   /** Projected (displayed) row count. */
   size: number;
   /** Rewinds applied since the history was created. */

packages/server/src/server/agent/timeline/internal/memory-backend.ts

@@ -283,6 +283,7 @@ export class MemoryTimelineBackend implements AgentTimelineBackend {
   private describe(state: MemoryTimelineState): TimelineHead {
     return {
       epoch: state.epoch,
+      seq: state.nextSeq - 1,
       size: state.projection.size,
       retirementCount: state.retirementCount,
       rewind: state.rewind ? { ...state.rewind } : null,