docs(agents): Protocol Domains table names marketplace/ where cloud/ used to be

The Protocol Domains table in AGENTS.md still listed a `Cloud` / `cloud/`
row after the cloud control-plane contracts left the open-source spec and
the package & marketplace format moved to `packages/spec/src/marketplace/`,
published as `@objectstack/spec/marketplace`. The row named a directory the
tree no longer has and omitted the one that replaced it.

Replace it with a `Marketplace` / `marketplace/` row at the same
alphabetical position (after `Kernel`), described from the directory's
actual modules. One line in, one line out — 1075 lines before and after.

Co-authored-by: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MCLBsUgfykL74aU716rzVK

Author: claude

AGENTS.md

@@ -654,7 +654,7 @@ content/docs/     # 📝 Docs content
 | `Identity` | `identity/` | User, Organization, Profile |
 | `Security` | `security/` | Permission, Role, Policy |
 | `Kernel` | `kernel/` | Plugin lifecycle (PluginContext) |
-| `Cloud` | `cloud/` | Multi-tenant, deployment, environment |
+| `Marketplace` | `marketplace/` | Package, Version, Listing, Install, Template |
 | `QA` | `qa/` | Test, validation |
 | `Contracts` | `contracts/` | Cross-package interfaces |
 | `Integration` | `integration/` | External integrations |