fix: preserve session continuity for timer/heartbeat wakes

Timer wakes had no taskKey, so they couldn't use agentTaskSessions for
session resume. Adds a synthetic __heartbeat__ task key for timer wakes
so they participate in the full session system.

Includes 6 dedicated unit tests for deriveTaskKeyWithHeartbeatFallback.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/src/__tests__/heartbeat-workspace-session.test.ts

@@ -4,6 +4,7 @@ import { sessionCodec as codexSessionCodec } from "@paperclipai/adapter-codex-lo
 import { resolveDefaultAgentWorkspaceDir } from "../home-paths.js";
 import {
   buildExplicitResumeSessionOverride,
+  deriveTaskKeyWithHeartbeatFallback,
   formatRuntimeWorkspaceWarningLog,
   prioritizeProjectWorkspaceCandidatesForRun,
   parseSessionCompactionPolicy,
@@ -184,6 +185,34 @@ describe("shouldResetTaskSessionForWake", () => {
   });
 });
 
+describe("deriveTaskKeyWithHeartbeatFallback", () => {
+  it("returns explicit taskKey when present", () => {
+    expect(deriveTaskKeyWithHeartbeatFallback({ taskKey: "issue-123" }, null)).toBe("issue-123");
+  });
+
+  it("returns explicit issueId when no taskKey", () => {
+    expect(deriveTaskKeyWithHeartbeatFallback({ issueId: "issue-456" }, null)).toBe("issue-456");
+  });
+
+  it("returns __heartbeat__ for timer wakes with no explicit key", () => {
+    expect(deriveTaskKeyWithHeartbeatFallback({ wakeSource: "timer" }, null)).toBe("__heartbeat__");
+  });
+
+  it("prefers explicit key over heartbeat fallback even on timer wakes", () => {
+    expect(
+      deriveTaskKeyWithHeartbeatFallback({ wakeSource: "timer", taskKey: "issue-789" }, null),
+    ).toBe("issue-789");
+  });
+
+  it("returns null for non-timer wakes with no explicit key", () => {
+    expect(deriveTaskKeyWithHeartbeatFallback({ wakeSource: "on_demand" }, null)).toBeNull();
+  });
+
+  it("returns null for empty context", () => {
+    expect(deriveTaskKeyWithHeartbeatFallback({}, null)).toBeNull();
+  });
+});
+
 describe("buildExplicitResumeSessionOverride", () => {
   it("reuses saved task session params when they belong to the selected failed run", () => {
     const result = buildExplicitResumeSessionOverride({