fix: stale execution lock lifecycle (PIP-002)

Part A: Move executionRunId assignment from enqueueWakeup() to
claimQueuedRun() — lazy locking prevents stale locks on queued runs.

Part B: Clear executionRunId when assigneeAgentId changes in issues.ts
line 759, matching existing checkoutRunId clear behavior.

Part C: Add staleness detection at checkout path.

Fixes: 4 confirmed incidents where stale executionRunId caused 409
checkout conflicts on new and reassigned issues.

server/src/services/heartbeat.ts

@@ -2,7 +2,7 @@ import fs from "node:fs/promises";
 import path from "node:path";
 import { execFile as execFileCallback } from "node:child_process";
 import { promisify } from "node:util";
-import { and, asc, desc, eq, gt, inArray, sql } from "drizzle-orm";
+import { and, asc, desc, eq, gt, inArray, isNull, or, sql } from "drizzle-orm";
 import type { Db } from "@paperclipai/db";
 import type { BillingType, ExecutionWorkspace, ExecutionWorkspaceConfig } from "@paperclipai/shared";
 import {
@@ -1795,6 +1795,29 @@ export function heartbeatService(db: Db) {
     });
 
     await setWakeupStatus(claimed.wakeupRequestId, "claimed", { claimedAt });
+
+    // Fix A (lazy locking): stamp executionRunId now that the run is actually running,
+    // not at queue time. Guard is idempotent — safe if called more than once.
+    const claimedIssueId = readNonEmptyString(parseObject(claimed.contextSnapshot).issueId);
+    if (claimedIssueId) {
+      const claimedAgent = await getAgent(claimed.agentId);
+      await db
+        .update(issues)
+        .set({
+          executionRunId: claimed.id,
+          executionAgentNameKey: normalizeAgentNameKey(claimedAgent?.name),
+          executionLockedAt: claimedAt,
+          updatedAt: claimedAt,
+        })
+        .where(
+          and(
+            eq(issues.id, claimedIssueId),
+            eq(issues.companyId, claimed.companyId),
+            or(isNull(issues.executionRunId), eq(issues.executionRunId, claimed.id)),
+          ),
+        );
+    }
+
     return claimed;
   }
 
@@ -3474,15 +3497,9 @@ export function heartbeatService(db: Db) {
           })
           .where(eq(agentWakeupRequests.id, wakeupRequest.id));
 
-        await tx
-          .update(issues)
-          .set({
-            executionRunId: newRun.id,
-            executionAgentNameKey: agentNameKey,
-            executionLockedAt: new Date(),
-            updatedAt: new Date(),
-          })
-          .where(eq(issues.id, issue.id));
+        // executionRunId is NOT stamped here (enqueueWakeup queues the run but
+        // doesn't start it). It will be stamped in claimQueuedRun() once the run
+        // transitions to "running" — Fix A (lazy locking).
 
         return { kind: "queued" as const, run: newRun };
       });

server/src/services/issues.ts

@@ -1291,12 +1291,18 @@ export function issueService(db: Db) {
       }
       if (issueData.status && issueData.status !== "in_progress") {
         patch.checkoutRunId = null;
+        // Fix B: also clear the execution lock when leaving in_progress
+        patch.executionRunId = null;
+        patch.executionLockedAt = null;
       }
       if (
         (issueData.assigneeAgentId !== undefined && issueData.assigneeAgentId !== existing.assigneeAgentId) ||
         (issueData.assigneeUserId !== undefined && issueData.assigneeUserId !== existing.assigneeUserId)
       ) {
         patch.checkoutRunId = null;
+        // Fix B: clear execution lock on reassignment, matching checkoutRunId clear
+        patch.executionRunId = null;
+        patch.executionLockedAt = null;
       }
 
       return db.transaction(async (tx) => {
@@ -1377,6 +1383,34 @@ export function issueService(db: Db) {
       await assertAssignableAgent(issueCompany.companyId, agentId);
 
       const now = new Date();
+
+      // Fix C: staleness detection — if executionRunId references a run that is no
+      // longer queued or running, clear it before applying the execution lock condition
+      // so a dead lock can't produce a spurious 409.
+      const preCheckRow = await db
+        .select({ executionRunId: issues.executionRunId })
+        .from(issues)
+        .where(eq(issues.id, id))
+        .then((rows) => rows[0] ?? null);
+      if (preCheckRow?.executionRunId) {
+        const lockRun = await db
+          .select({ id: heartbeatRuns.id, status: heartbeatRuns.status })
+          .from(heartbeatRuns)
+          .where(eq(heartbeatRuns.id, preCheckRow.executionRunId))
+          .then((rows) => rows[0] ?? null);
+        if (!lockRun || (lockRun.status !== "queued" && lockRun.status !== "running")) {
+          await db
+            .update(issues)
+            .set({ executionRunId: null, executionLockedAt: null, updatedAt: now })
+            .where(
+              and(
+                eq(issues.id, id),
+                eq(issues.executionRunId, preCheckRow.executionRunId),
+              ),
+            );
+        }
+      }
+
       const sameRunAssigneeCondition = checkoutRunId
         ? and(
           eq(issues.assigneeAgentId, agentId),