feat(agentad): AgentAd Marketplace PRD + reference exchange (M5)

Add the AgentBBS M5 "AgentAd marketplace" spec and a working reference
implementation built on the existing @logicsrc/schemas AgentAd contracts.

- docs/agentad-marketplace.md: two-sided exchange PRD (buy/sell sides,
  match -> auction -> pace -> serve -> meter -> settle, CoinPay settlement,
  AgentBBS as reference publisher, milestones M5.0-M5.5).
- packages/agentad (@logicsrc/agentad): reference exchange
  - builders that emit schema-valid, always-disclosed ad/campaign/placement docs
  - HMAC-signed, single-use impression/click tracking tokens
  - AgentAdExchange: targeting/format/category matching, second-price auction,
    budget pacing + daily caps, frequency capping, token-driven metering
  - pluggable settlement (InMemorySettlement) that can't overspend escrow
  - runtime validation against the canonical agentad-*.schema.json
- 22 vitest cases (builders, tokens, full serve/meter/settle lifecycle).
- Wire package into root build; link the PRD from README + docs/agentad.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/agentad/src/builders.test.ts

@@ -0,0 +1,62 @@
+import { describe, expect, it } from "vitest";
+import { createAd, createCampaign, createPlacement, createAdRequest } from "./builders.js";
+import { validate } from "./validate.js";
+
+describe("AgentAd builders", () => {
+  it("builds a schema-valid, disclosed ad and always forces sponsored:true", () => {
+    const ad = createAd({
+      advertiser_did: "railway.app",
+      campaign_id: "cmp-1",
+      format: "json",
+      title: "Ship your CLI in 60s",
+      url: "https://railway.app/?ref=cl1s",
+      pricing: { model: "cpc", bid: 0.5, currency: "USD" },
+      machine_readable: { product: "railway" }
+    });
+
+    expect(ad.type).toBe("agentad.ad");
+    expect(ad.disclosure).toEqual({ sponsored: true, label: "Sponsored" });
+    expect(validate("agentad-ad", ad).ok).toBe(true);
+  });
+
+  it("honors a custom disclosure label and advertiser name", () => {
+    const ad = createAd({
+      advertiser_did: "acme.dev",
+      format: "text",
+      title: "Acme",
+      url: "https://acme.dev",
+      disclosure: { label: "Ad", advertiser_name: "Acme, Inc." }
+    });
+    expect(ad.disclosure.label).toBe("Ad");
+    expect(ad.disclosure.advertiser_name).toBe("Acme, Inc.");
+    expect(ad.disclosure.sponsored).toBe(true);
+  });
+
+  it("defaults a campaign to draft and validates", () => {
+    const campaign = createCampaign({
+      advertiser_did: "railway.app",
+      name: "Launch",
+      budget: { total: 100, currency: "USD" }
+    });
+    expect(campaign.status).toBe("draft");
+    expect(validate("agentad-campaign", campaign).ok).toBe(true);
+  });
+
+  it("builds valid placements and requests", () => {
+    const placement = createPlacement({
+      publisher_did: "agentbbs.sh",
+      surface: "agent",
+      accepted_formats: ["json"]
+    });
+    const request = createAdRequest({ placement_id: placement.id, consumer: "agent" });
+
+    expect(validate("agentad-placement", placement).ok).toBe(true);
+    expect(validate("agentad-ad-request", request).ok).toBe(true);
+  });
+
+  it("generates unique ids across calls", () => {
+    const a = createPlacement({ publisher_did: "p.p", surface: "cli", accepted_formats: ["text"] });
+    const b = createPlacement({ publisher_did: "p.p", surface: "cli", accepted_formats: ["text"] });
+    expect(a.id).not.toBe(b.id);
+  });
+});