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/package.json

@@ -0,0 +1,19 @@
+{
+  "name": "@logicsrc/agentad",
+  "version": "0.1.0",
+  "description": "AgentAd Marketplace reference exchange: campaign/placement registration, second-price auction, budget pacing, signed impression/click metering, and CoinPay-style settlement over the @logicsrc AgentAd schemas.",
+  "type": "module",
+  "main": "./dist/index.js",
+  "types": "./dist/index.d.ts",
+  "scripts": {
+    "build": "tsc -p tsconfig.json",
+    "test": "vitest run src"
+  },
+  "dependencies": {
+    "ajv": "^8.17.1",
+    "ajv-formats": "^3.0.1"
+  },
+  "devDependencies": {
+    "vitest": "^4.0.8"
+  }
+}