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>

package-lock.json

@@ -1390,6 +1390,10 @@
       "resolved": "packages/account-core",
       "link": true
     },
+    "node_modules/@logicsrc/agentad": {
+      "resolved": "packages/agentad",
+      "link": true
+    },
     "node_modules/@logicsrc/agentstack": {
       "resolved": "packages/agentstack",
       "link": true
@@ -6486,6 +6490,17 @@
         "vitest": "^4.0.8"
       }
     },
+    "packages/agentad": {
+      "name": "@logicsrc/agentad",
+      "version": "0.1.0",
+      "dependencies": {
+        "ajv": "^8.17.1",
+        "ajv-formats": "^3.0.1"
+      },
+      "devDependencies": {
+        "vitest": "^4.0.8"
+      }
+    },
     "packages/agentstack": {
       "name": "@logicsrc/agentstack",
       "version": "0.1.0",