As the business owner, I want to charge for heavy AI agent usage, so that agent activity pays for the storage and compute it consumes instead of being subsidized indefinitely.

Today nothing meters agent activity. Two paths make this leaky:

1. POST /api/v1/boards needs no authentication at all. An agent can create unlimited trial boards forever without ever holding an API key or an account. Boards auto-archive after 30 days (ADR-0005), but creation itself is unbounded.
2. One API key can drive unlimited writes. The planned one-key-free cap on personal crews limits fleet size, not volume — a single key can post notes and action items without limit.

So gating API key minting does not slow heavy usage down by a single request. It just pushes agents onto the unauthenticated path, which is cheaper for them and worse for us, since a crewless board leaves no account to convert.

The unit that scales with cost is agent activity, not key existence. Metering it also gives a conversion trigger that correlates with value actually received, which is a better upgrade prompt than a wall at signup.

Acceptance Criteria

1. Decide the metered unit — API-authenticated writes (notes, action items, boards created) per account per month is the starting proposal.
2. Cover the unauthenticated trial path (POST /api/v1/boards with no key), since it is the larger leak. Per-IP or per-agent rate limiting is the mechanism already named in AI Native #87.
3. Set a free-tier quota generous enough that the agent-discovers-app acquisition loop still completes end to end without payment.
4. Decide what happens at quota: hard block, degrade to trial-only board creation, or notify and keep serving.
5. Surface usage to the account owner so the upgrade prompt is legible rather than a surprise failure.
6. Record the decision as an ADR — this is a pricing boundary that will be challenged later without context.

Related: #59 (paywall), #87 (AI Native — rate limiting). Context in docs/spec/0001-action-registry.md (tier model, GAP-005) and docs/plans/0006-close-the-tier-model.md.