Feature request: Include parent sub-agent context in hook stdin

Feature Request

Include sub-agent context in hook stdin so that tool calls from background sub-agents can be attributed to their parent.

Current Behavior

When a background sub-agent (launched via the Agent tool with run_in_background: true) executes tools like Grep, Read, or Glob, the PreToolUse hook fires with:

{
  "tool_name": "Grep",
  "tool_input": { "pattern": "..." }
}

No field identifies which sub-agent triggered this tool call.

Requested Behavior

Include the parent sub-agent's tool_call_id (or an equivalent session/sub-agent identifier) in the hook stdin:

{
  "tool_name": "Grep",
  "tool_input": { "pattern": "..." },
  "parent_tool_call_id": "toolu_01G4DRTnHw4vqwHcSddaHn4u"
}

Use Case

Hooks that need to track or log sub-agent activity cannot distinguish which concurrent background sub-agent triggered a given tool call. With a single background sub-agent this is unambiguous, but with multiple concurrent background sub-agents there is no way to attribute tool calls to the correct parent.

Context

• Background sub-agent tool calls don't appear on the ACP wire — only the parent Agent/Task tool call does
• The hook system is the only visibility path for background sub-agent tool activity
• The SubagentStop hook already fires per sub-agent — this request extends that attribution to PreToolUse and PostToolUse