Add frontend-facing agent access resolution abilities

[chubes4]

Summary

frontend-agent-chat migration PR Extra-Chill/frontend-agent-chat#9 exposed that Agents API has the generic access contracts, but no frontend-friendly helper/ability for common product questions:

• Can the current principal chat with this agent?
• Which agents can the current principal access?
• What display metadata should a client render for each accessible agent?

Agents API already has the lower-level pieces:

• WP_Agent_Access_Grant
• WP_Agent_Access_Store
• WP_Agent_Authorization_Policy
• WP_Agent_WordPress_Authorization_Policy::can_access_agent()

But a standalone frontend plugin still has to know host-specific storage, currently Data Machine's PermissionHelper, to answer these questions.

Proposed API

Add canonical abilities and/or helpers:

wp_agent_current_user_can_access_agent( string $agent_slug, string $minimum_role = 'viewer' ): bool
wp_agent_list_accessible_agents( string $minimum_role = 'viewer' ): array

Ability equivalents:

agents/can-access-agent
agents/list-accessible-agents

agents/list-accessible-agents should return registered agent metadata suitable for frontend selectors:

array(
  'agents' => array(
    array(
      'slug' => 'site-assistant',
      'label' => 'Site Assistant',
      'description' => '...',
      'role' => 'viewer',
      'meta' => array(),
    ),
  ),
)

Host Extension Point

Hosts should provide concrete stores via a filter rather than consumers importing host classes:

apply_filters( 'wp_agent_access_store', null, $context )
apply_filters( 'wp_agent_authorization_policy', null, $context )

Data Machine can return its concrete access store or adapter. Other products can do the same.

Acceptance Criteria

• A frontend plugin can check access to an agent without importing Data Machine classes.
• A frontend plugin can list accessible agents for the current user/principal.
• Registered WP_Agent definitions and host-materialized grants compose cleanly.
• Data Machine can keep its current storage while exposing it through Agents API contracts.

Related

• Migrate frontend chat to Agents API Extra-Chill/frontend-agent-chat#9
• Add generic agent access grants #14
• Scope model: promote workspace-scoped memory, permissions, transcripts, and approvals #55