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>

docs/agentad.md

@@ -63,3 +63,9 @@ The `consumer` field on a request changes how an ad is delivered:
 
 [cl1s.tech](https://github.com/profullstack/cl1s.tech) is the hosted AgentAd
 network: SDK, CLI, and ad-serving service that consume these schemas.
+
+## Marketplace
+
+The two-sided exchange built on these contracts — advertiser campaigns, publisher
+placements, auction, metering, and CoinPay settlement — is specified in
+[AgentAd Marketplace (PRD)](./agentad-marketplace.md) (AgentBBS milestone M5).