Hook input should include context to distinguish agent vs subagent

[sfriedenberg-etsy]

PreToolUse hook input JSON currently contains hook_event_name, tool_name, tool_input, cwd, and session_id. There's no field indicating whether the tool call originates from the main agent or a subagent.

This matters for hook handlers that want to provide different guidance depending on context. For example, when denying a Bash call and redirecting to an MCP resource, the main agent can read the resource directly via ReadMcpResourceTool, but subagents must use a resource-read tool wrapper since they don't have access to MCP resources.

Currently we include both instructions in every denial message, which is noisy. A field like agent_type or is_subagent in the hook input would allow targeted messages.

[github-actions]

Found 3 possible duplicate issues:

1. Feature Request: Add agent context to hook events for filtering notifications by agent type #25134
2. Agent Context Detention in Hook Events #6885
3. [FEATURE] 1. Agent Hierarchy in Hook Events, 2. Intermediate Text Output Hook, 3. SubagentStart Hook #14859

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[github-actions]

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