feat(web): real linked Signal projection from signalIssueAttachments

Replace the deliberate signalCount of 0 with authenticated linked-Signal
data from the Lane A signalIssueAttachments relation.

- Add listLinkedSignalsForProject query (user-authenticated via
  requireProjectMember) returning issue-grouped LinkedSignalView data
  using the existing by_issue index. One query serves both card counts
  and expanded detail.
- Update work-unit-projection builders to accept linkedSignals param;
  LinkedSignal type with signalId, title, summary, sourceCount, createdAt.
- Feed real attachment data into collapsed card signalCount and expanded
  detail linked-signal list with provenance (source count, relative time).
- Add signal.attached event handler to activity timeline projection.
- 32 tests including 4 linked-signal projection tests, 1 signal.attached
  activity mapping test, and cross-issue signal isolation test proving
  issue A cannot inherit issue B's linked signals.

packages/backend/convex/signals.ts

@@ -9,7 +9,7 @@ import {
   type QueryCtx,
   query,
 } from "./_generated/server";
-import { requireOrganizationMember } from "./authz";
+import { requireOrganizationMember, requireProjectMember } from "./authz";
 
 const PROBLEM_STATEMENT = v.object({
   title: v.string(),
@@ -435,3 +435,70 @@ export const get = query({
     };
   },
 });
+
+// ---------------------------------------------------------------------------
+// Linked Signal query for the Work OS surface.
+// ---------------------------------------------------------------------------
+
+export interface LinkedSignalView {
+  readonly signalId: Id<"signals">;
+  readonly title: string;
+  readonly summary: string;
+  readonly sourceCount: number;
+  readonly createdAt: number;
+}
+
+/**
+ * List all Signal-issue attachments for a project, grouped by issue id.
+ * User-authenticated: the caller must be a member of the project organization.
+ * Used by the Work OS card list to show per-issue linked Signal counts.
+ */
+export const listLinkedSignalsForProject = query({
+  args: {
+    projectId: v.id("projects"),
+  },
+  handler: async (ctx, args): Promise<Record<string, LinkedSignalView[]>> => {
+    const { organizationId } = await requireProjectMember(ctx, args.projectId);
+    const issues = await ctx.db
+      .query("projectIssues")
+      .withIndex("by_project_and_number", (q) =>
+        q.eq("projectId", args.projectId)
+      )
+      .take(100);
+    const result: Record<string, LinkedSignalView[]> = {};
+    for (const issue of issues) {
+      const attachments = await ctx.db
+        .query("signalIssueAttachments")
+        .withIndex("by_issue", (q) => q.eq("issueId", issue._id))
+        .collect();
+      if (attachments.length === 0) {
+        continue;
+      }
+      const signals = await Promise.all(
+        attachments.map(async (attachment) => {
+          const signal = await ctx.db.get(attachment.signalId);
+          if (!signal || signal.organizationId !== organizationId) {
+            return null;
+          }
+          const sources = await ctx.db
+            .query("signalSources")
+            .withIndex("by_signal_and_ordinal", (q) =>
+              q.eq("signalId", signal._id)
+            )
+            .collect();
+          return {
+            createdAt: signal.createdAt,
+            signalId: signal._id,
+            sourceCount: sources.length,
+            summary: signal.problemStatement.summary,
+            title: signal.problemStatement.title,
+          } satisfies LinkedSignalView;
+        })
+      );
+      result[String(issue._id)] = signals.filter(
+        (entry): entry is LinkedSignalView => entry !== null
+      );
+    }
+    return result;
+  },
+});