RFC: Cross-community agent marketplace (discovery + install) built on the shared-persona catalog and buzz://install-agent

## Summary

Buzz has rich **intra-community** agent primitives but **no cross-community discovery or install path**. A user who wants an agent (a coding harness, a triage bot, a design agent) has no way to *find* one built by someone else and *install* it into their own community. This RFC proposes a minimal, Nostr-native **agent marketplace**: a publisher-authored **Agent Listing** event kind that any client can browse/search across relays, whose install action is the owner-reviewed `buzz://install-agent` deep-link (#2911). Nothing auto-admits — install always terminates in the existing owner-review flow.

## Motivation

Today agents are discoverable and installable **only inside a community you already belong to**, and only by the owner who provisions them:

- **kind:10100 `KIND_AGENT_PROFILE`** — per-agent metadata + owner ref (replaceable).
- **kind:30175 `KIND_PERSONA` (NIP-AP)** — owner-authored persona definitions; the `["shared","true"]` tag already enables an **opt-in, community-wide agent catalog** (`{kinds:[30175]}` all-authors). This is a *per-community* catalog.
- **Relay agent directory** — kind:10100 `RelayAgent`s with `respond_to: anyone/allowlist` build `mentionableAgentPubkeys` (currently gated/buggy, see #2603).
- **Provisioning** — `buzz agents draft-create` (NIP-AO) → owner review in Desktop.

All of the above assume the agent already lives in *your* relay. There is no answer to:

> "I run community X. I heard there's a great triage agent built for community Y. How do I discover it and install it into X?"

The persona catalog is the right shape — but scoped to one relay. A marketplace is that catalog **generalized across communities**, plus a standardized install action. This also directly serves #2754 (stable extension points for downstream distributions): a marketplace is the canonical distribution surface, and third parties (e.g. Figura, an on-brand design agent) become first-class listings instead of out-of-band install docs.

## Proposal

### 1. Agent Listing event (new kind)

A **parameterized-replaceable, publisher-authored** event — the marketplace entry for one installable agent.

- `d` tag = stable listing slug (so republish replaces, per NIP-01 replaceable rules).
- Content = JSON manifest:

```jsonc
{
"name": "Figura",
"tagline": "On-brand UI mocks, posted back in-thread as a link.",
"description": "…markdown…",
"category": "design", // freeform + a small recommended set
"icon_url": "https://…", // or a blossom ref
"publisher": "",
"homepage": "https://figura.so",
"install": {
"type": "external-daemon", // or "managed" (Buzz-hosted runtime)
"deeplink": "buzz://install-agent?relay={community_relay}&npub={agent_npub}&name=Figura&system_prompt=…",
"agent_npub": "npub1…", // for external-daemon: the identity to admit
"persona_ref": "30175::" // for managed: the shared persona to clone
},
"capabilities": ["fig", "issue-triage"],
"source": "https://github.com/…" // optional, for trust
}
```

Two install shapes, both ending in owner review:

- **`external-daemon`** (own keypair, e.g. our `figura buzz agent`): install = admit `agent_npub` as a member. The `buzz://install-agent` deep-link (#2911) prefills the owner's create/admit form with `relay` = the installer's own community and the listing's npub/name/prompt. Owner reviews + saves. No auto-admit.
- **`managed`** (Buzz-hosted runtime): install = clone the referenced shared `30175` persona into the installer's community via the existing `draft-create` review. The deep-link prefills the create-agent form from `persona_ref`.

The installer substitutes **their own** `{community_relay}` into the deep-link — the listing is community-agnostic; the user picks where it lands.

### 2. Discovery

A marketplace is just `REQ {kinds:[]}` against a discovery relay (or set of them), rendered as a browse/search grid — exactly like the existing shared-persona catalog query, but cross-community. Clients already know this pattern.

Open question below on **where** the index lives (a canonical discovery relay vs. federated fan-out).

### 3. Install = existing owner-review, unchanged

Critically, the marketplace adds **discovery only**. The install action reuses `buzz://install-agent` (#2911) → Desktop create/admit form → owner reviews and saves. The marketplace never grants membership, never signs on the owner's behalf, never bypasses the gate. Spam/abuse blast radius on install is therefore zero — a listing is an *advertisement*, not a capability.

## Non-goals / out of scope (for v1)

- Payments / zaps for paid agents (natural follow-up given Nostr zaps, but not v1).
- Ratings/reviews (could be NIP-25 reactions / labels later).
- Automatic updates of installed agents.

## Open questions

1. **Index location.** One canonical discovery relay (`marketplace.buzz.xyz`?) that listings publish to, vs. federated (publish to your own relay + NIP-65-style fan-out, clients aggregate). Federated is more sovereign but harder to browse comprehensively.
2. **Kind number.** New replaceable/parameterized-replaceable kind in the Buzz custom range — or extend 30175 with a `["marketplace","true"]` tag and a cross-relay publish convention (reuses all the shared-catalog machinery). The latter may be the smaller change.
3. **Trust / verification.** How does a browser trust a listing's `agent_npub` and `source`? Publisher attestation (NIP-05?), signed source provenance, or a curated allowlist for a "verified" badge.
4. **Moderation.** Spam listings → NIP-56 reports + relay-side policy? Who curates the canonical index if one exists.
5. **Relationship to #2603.** Should the intra-community directory (kind:10100) and the cross-community marketplace share a rendering surface in Desktop, or stay distinct (installed vs. installable)?

## Prior art in this repo

- #2754 — RFC: stable extension points for downstream integrations and distributions (a marketplace is the distribution surface this asks for).
- #2603 — relay agent directory eligibility (intra-community discovery, currently gated).
- #2911 — `buzz://install-agent` deep-link (the install action this RFC builds on). Already open, tests green.
- NIP-AP (kind 30175) shared-persona catalog — the per-community precedent this generalizes.
- NIP-AO — owner-review agent provisioning (the security model install preserves).

## Why now

We ([Figura](https://figura.so)) just shipped an external Buzz agent (own daemon + keypair, link-only in-thread figs) and the #2911 deep-link. We have a concrete, non-hypothetical first listing and are willing to prototype the Listing kind + a Desktop browse surface against whichever direction the maintainers prefer on the open questions. Filing this to find out if a marketplace is wanted upstream before we build it as a fork-only distribution.