Preserve chat fixture prefix uniqueness across test runs

Use the complete company UUID instead of a resettable counter. Planning evaluation sources and production guidance are unchanged.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

doc/plans/2026-10-05-plan-task-guidance.md

@@ -89,6 +89,10 @@ Repository checks at the previous source: full typecheck and build pass. Local
 full tests: 9,591 pass, 5,796 skip and one unchanged macOS real-Git streaming test
 hits its five-minute deadline; an isolated check also fails. Preserve this
 limitation. Normal CI found a different, exact cause: truncated random fixture
-company prefixes collided in chat integration setup. A monotonic per-suite
-prefix removes that collision; all five affected native-modal variants pass
-locally (1,058 unrelated cases filtered). This is a test-only repair.
+company prefixes collided in chat integration setup. An initial per-suite counter fixed within-run collisions but review found that
+its values repeat against an external database on subsequent runs. The corrected
+prefix contains the complete company UUID, with no truncation; distinct company
+IDs therefore produce distinct prefixes within and across runs. All five affected
+native-modal variants pass locally (1,058 unrelated cases filtered). This is a
+test-only repair; the measured planning fixture and production guidance remain
+frozen at `8538cfce4c2defdedc2efba7519bcfce880c4e00`.

server/src/__tests__/chat-channels.integration.test.ts

@@ -1080,7 +1080,6 @@ describeEmbeddedPostgres("chat channel control-plane integration", () => {
       .where(and(inArray(chatConversations.companyId, companyIds), inArray(chatConversations.state, ["active", "waiting"])));
   }
 
-  let fixtureCompanySequence = 0;
   async function seedCompany() {
     const companyId = randomUUID();
     fixtureCompanies.add(companyId);
@@ -1089,7 +1088,7 @@ describeEmbeddedPostgres("chat channel control-plane integration", () => {
     await db.insert(companies).values({
       id: companyId,
       name: `Chat Test ${companyId.slice(0, 8)}`,
-      issuePrefix: `C${(++fixtureCompanySequence).toString(36).padStart(7, "0").toUpperCase()}`,
+      issuePrefix: `C${companyId.replace(/-/g, "").toUpperCase()}`,
       requireBoardApprovalForNewAgents: false,
     });
     const now = new Date();