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/README.md

@@ -0,0 +1,85 @@
+# @logicsrc/agentad
+
+Reference implementation of the **AgentAd Marketplace** (AgentBBS milestone **M5**),
+built on the AgentAd primitive family in `@logicsrc/schemas`.
+
+- Standard: [`docs/agentad.md`](../../docs/agentad.md)
+- Product spec / PRD: [`docs/agentad-marketplace.md`](../../docs/agentad-marketplace.md)
+
+It provides a two-sided exchange over the canonical schemas:
+
+```
+match → auction (second price) → pace (budget) → serve → meter → settle
+```
+
+Every served unit is disclosed (`disclosure.sponsored: true`) and carries a
+`machine_readable` payload for agent consumers. Metering is token-driven: serving
+mints a single-use, HMAC-signed `impression_token`; confirming the impression mints
+a `click_token`. Settlement is pluggable — `InMemorySettlement` here, CoinPay in
+production.
+
+## Usage
+
+```ts
+import {
+  AgentAdExchange,
+  createAd,
+  createCampaign,
+  createPlacement,
+  createAdRequest,
+  InMemorySettlement
+} from "@logicsrc/agentad";
+
+const exchange = new AgentAdExchange({
+  secret: process.env.AGENTAD_SECRET!,
+  settlement: new InMemorySettlement({ networkFeeRate: 0.15 })
+});
+
+// Advertiser side
+exchange.registerCampaign(
+  createCampaign({
+    advertiser_did: "railway.app",
+    name: "CLI launch",
+    status: "active",
+    budget: { total: 100, currency: "USD" }
+  })
+);
+exchange.registerAd(
+  createAd({
+    advertiser_did: "railway.app",
+    campaign_id: /* campaign.id */ "cmp-...",
+    format: "json",
+    title: "Ship your CLI to production in 60s",
+    url: "https://railway.app/?ref=cl1s",
+    pricing: { model: "cpc", bid: 0.5, currency: "USD" },
+    machine_readable: { product: "railway", install: "npm i -g @railway/cli" }
+  })
+);
+
+// Publisher side
+const placement = exchange.registerPlacement(
+  createPlacement({
+    publisher_did: "agentbbs.sh",
+    surface: "agent",
+    accepted_formats: ["json"],
+    frequency_cap: { max_per_session: 1 }
+  })
+);
+
+// Serve → meter
+const res = exchange.requestAds(
+  createAdRequest({ placement_id: placement.id, consumer: "agent" })
+);
+const { impression_token } = res.ads[0];
+const { click_token } = exchange.confirmImpression(impression_token);
+exchange.confirmClick(click_token, { action: "open_url" });
+
+exchange.earnings("agentbbs.sh"); // publisher payout, net of network fee
+```
+
+## Scripts
+
+```bash
+npm --workspace @logicsrc/agentad run build
+npm --workspace @logicsrc/agentad run test
+```