feat(acp): propagate parent_tool_use_id through LoopbackEvent and SessionUpdate

[bug-ops]

Part of epic: #1007

Problem

When the orchestrator spawns a subagent, all events from that subagent are emitted flat alongside top-level agent events. IDEs cannot distinguish or nest subagent output.

Solution

Add parent_tool_use_id: Option<String> to LoopbackEvent variants and propagate it through to session_update._meta.claudeCode.parentToolUseId as per the Zed reference pattern.

Protocol shape (from Zed claude-agent-acp):

{
  "type": "agent_message_chunk",
  "_meta": {
    "claudeCode": {
      "parentToolUseId": "<uuid-or-null>"
    }
  }
}

Tasks

• Add parent_tool_use_id: Option<String> to all content-bearing LoopbackEvent variants in crates/zeph-acp/src/agent.rs
• Add parent_tool_use_id field to AgentContext (or new SubagentContext wrapper) in crates/zeph-core/src/agent/
• Inject parent id in orchestrator when spawning subagent loop
• In prompt() handler: read parent id from event, serialize into _meta.claudeCode.parentToolUseId
• Unit tests: verify field is null for top-level events and <uuid> for subagent events

Files

• crates/zeph-acp/src/agent.rs — LoopbackEvent enum + prompt() handler
• crates/zeph-core/src/agent/ — AgentContext + orchestrator dispatch