Agents API in-repo module: create core-style agents-api boundary

[chubes4]

Refs #1561, #1596, #1618.

Problem

Data Machine now contains Agents API-shaped vocabulary and contracts, but they are scattered across Data Machine product directories instead of living behind a core-style in-repo module boundary.

Current examples include:

• inc/Engine/Agents/register-agents.php
• inc/Engine/Agents/class-wp-agent.php
• inc/Engine/Agents/class-wp-agents-registry.php
• inc/Engine/AI/AgentConversationRequest.php
• inc/Engine/AI/AgentMessageEnvelope.php
• inc/Core/FilesRepository/AgentMemoryStoreInterface.php
• inc/Core/Database/Chat/ConversationTranscriptStoreInterface.php

The desired architecture is an in-repo agents-api module that is treated like WordPress core substrate while it still ships inside Data Machine.

Target shape

Create a Data Machine-owned in-repo module:

data-machine/
  agents-api/
    agents-api.php
    inc/
    tests/
  inc/
    ...Data Machine product code...

Data Machine loads agents-api first, then consumes it as product code.

Boundary rules

• agents-api must not import Data Machine product namespaces.
• Data Machine may import agents-api.
• agents-api owns WordPress-shaped public vocabulary such as wp_register_agent(), WP_Agent, WP_Agents_Registry, wp_agents_api_init, AgentMessageEnvelope, and runtime contracts.
• Data Machine keeps flows, pipelines, jobs, handlers, queues, retention, admin UI, pending actions, adjacent handler tools, and product adapters.

Acceptance criteria

• data-machine/agents-api exists.
• Data Machine loads the module before product runtime bootstraps.
• agents-api can run a bootstrap smoke without loading Data Machine product code.
• No agents-api file imports DataMachine\Core\Steps, DataMachine\Core\Database\Jobs, handlers, queues, retention, or admin UI.
• Existing Data Machine behavior remains unchanged.