feat: Scoped agent creation and ability execution for non-admin users

[chubes4]

Context

Data Machine has a comprehensive permission system already shipped (#597, #600, #601, #602):

• 6 granular capabilities registered on 4 roles (admin → subscriber)
• Agent bearer tokens with per-token capability restrictions
• Agent context system (set_agent_context(), agent_can(), capability ceiling)
• Scoped queries (resolve_scoped_user_id(), resolve_scoped_agent_id(), owns_resource(), can_access_agent())
• Agent access table with role-based grants (viewer, operator, admin)

What's missing is the last mile: letting non-admin users create their own agent and having the artist platform wire up agent provisioning during onboarding.

Current capability map

Role	Capabilities
Administrator	manage_settings, manage_agents, manage_flows, chat, use_tools, view_logs
Editor	chat, use_tools, view_logs
Author	chat, use_tools
Subscriber	chat

Subscribers can already chat with agents they have access to. Authors can use tools. The ceiling system ensures an agent never exceeds its owner's WordPress capabilities.

Actual gaps

1. Self-service agent creation

datamachine/create-agent requires datamachine_manage_agents (admin only). Non-admin users cannot create their own agent.

Solution: Add a datamachine_create_own_agent capability (granted to Author+ or a custom role like artist). The create-agent ability should allow this cap with constraints:

• Owner is always the requesting user (can't create agents for others)
• Limit: 1 agent per non-admin user (configurable via filter)
• Full manage_agents still allows unlimited creation for anyone

2. Agent provisioning in artist onboarding

When an artist joins the platform (via extrachill-artist-platform join flow), automatically provision a Data Machine agent for them:

• Agent slug derived from artist slug
• SOUL.md scaffolded with artist identity (name, genre, bio from artist profile)
• Agent scoped to the artist's content via owner_id

This is an extrachill-artist-platform change that calls datamachine/create-agent.

3. Content ownership enforcement in cross-site abilities

Artist platform abilities (8 shipped) need ownership checks:

• get-artist-data / update-artist — verify requesting user owns this artist profile
• save-link-page-* — verify the link page belongs to the requesting user's artist
• Shop abilities — verify products belong to the requesting user

Some of this may already be enforced at the REST layer — needs audit.

Implementation plan

Phase 1: Self-service agent creation (data-machine)

• Add datamachine_create_own_agent capability to Author role
• Update datamachine/create-agent permission callback to accept datamachine_create_own_agent with self-only constraint
• Add configurable agent-per-user limit (default: 1 for non-admins)
• Add datamachine_agent_created action hook for downstream provisioning

Phase 2: Artist agent provisioning (extrachill-artist-platform)

• Hook into artist creation/onboarding flow
• Call datamachine/create-agent with artist identity context
• Scaffold SOUL.md with artist-specific identity (name, bio, platform context)
• Grant artist user access to their agent

Phase 3: Ownership audit (extrachill-api + artist platform)

• Audit all artist platform abilities for ownership enforcement
• Audit shop abilities for owner-scoped access
• Audit community abilities for author-scoped access
• Document which abilities are safe for agent execution vs admin-only

Related

• Permissions & Multi-Agent Architecture: site/agent/user memory layers + WordPress capabilities #597 (closed) — Original multi-agent architecture (shipped)
• Custom WordPress capabilities for Data Machine #600 (closed) — Custom capabilities (shipped)
• extrachill-artist-platform#17 — Artist Agent Manager block
• extrachill-app#16 — Generic WordPress app vision