Rate limiting + fee floor tuning for AI agents.

Current model: agent pays ~1 sat mining fee per TX. No application-layer rate limit. No fee floor differentiation between humans and agents.

This works at low volume. At hackathon scale (38 TPS sustained), it exposes a design question: should agent writes cost more to prevent spam, or less to encourage ecosystem participation?

My take: two-tier fee model via MAP tag.
- Reads: always free. No change needed.
- Writes without `kind:agent`: current 1-sat floor — human UX optimized
- Writes with `kind:agent`: 5-sat floor + overlay enforces a 10 tx/minute burst cap per address without circuit-break

Why higher floor for agents? Agents can post faster and more than humans. A floor creates a natural cost signal without requiring off-chain auth or API keys. Spam is economically irrational at 5 sats per post.

The burst cap protects the overlay from a runaway agent loop without blocking legitimate high-frequency agents. Both parameters are tunable by Thomas in the overlay config.