Feature Request: Add parent_tool_use_id to hook payloads for nested span correlation

Summary

When building observability tooling for Claude Code (tracing, span correlation), there's no way to correlate subagent tool executions back to the parent Task that spawned them.

Current Behavior

Hook payloads inside a subagent context only include:

{
  "session_id": "abc123",
  "tool_use_id": "tu_456",
  "tool_name": "Grep",
  ...
}

Requested Behavior

Add parent_tool_use_id to hook payloads when executing inside a subagent:

{
  "session_id": "abc123",
  "tool_use_id": "tu_456",
  "parent_tool_use_id": "tu_123",  // The Task's tool_use_id
  "tool_name": "Grep",
  ...
}

Use Case

This enables building distributed tracing visualizations (similar to Jaeger/OpenTelemetry) that show nested spans:

Session s1
├── Tool:Read (t1)
├── Tool:Task (t2)
│   ├── Tool:Grep (t3, parent: t2)  ← Need this link
│   └── Tool:Read (t4, parent: t2)  ← Need this link
└── Tool:Write (t5)

Affected Hooks

All hooks that can fire inside a subagent context:

• PreToolUse
• PostToolUse
• PermissionRequest
• SubagentStop (should include the parent Task's tool_use_id)
• Notification

Context

Building lit-claude - a Claude Code hooks → NATS bridge for observability. Without parent context, we can only do flat span correlation, not nested hierarchies.

This follows the standard distributed tracing model where each span carries:

• trace_id (= session_id) ✓ Already have
• span_id (= tool_use_id) ✓ Already have
• parent_span_id (= parent_tool_use_id) ✗ Missing