feat: add multi-agent support via dm-agent-sync plugin

[chubes4]

Summary

Adds generic multi-agent support so that all Data Machine agents automatically surface in OpenCode's agent switcher with full identity, memory, and tool context.

What's included

1. kimaki/plugins/dm-agent-sync.ts — New OpenCode plugin that:

• Queries wp datamachine agents list for all active agents at session start
• Resolves file paths via wp datamachine agent paths per agent
• Registers each as an OpenCode agent with layered prompt (AGENTS.md → SITE.md → SOUL.md → MEMORY.md → USER.md)
• Maps agent_config.default_model to OpenCode model config
• Maps agent_config.tool_policy to OpenCode tool permissions
• Respects user overrides in opencode.json (doesn't clobber explicit model choices)
• No-ops gracefully when Data Machine is unavailable

2. setup.sh Phase 7 update — Includes dm-agent-sync.ts in the generated plugin array alongside dm-context-filter.ts

Architecture

Data Machine (brain)          →  Agent database, SOUL.md, MEMORY.md, tool policies
    ↓ WP-CLI queries
dm-agent-sync.ts (plugin)     →  Reads DM agents, registers in OpenCode
    ↓ config hook
OpenCode (runtime)            →  User sees all agents in agent switcher

Backward compatibility

• Zero agents: plugin is a silent no-op
• One agent: updates build/plan prompts to ensure file paths are current
• Multiple agents: registers each by slug + populates build/plan defaults from first agent

Depends on

• feat: add agent lifecycle hooks (created, updated, deleted) data-machine#1023 (agent lifecycle hooks — nice to have, not blocking)