Feature Request: Per-agent extended thinking configuration

Summary

Allow configuring extended thinking (thinking budget) on a per-subagent basis when using the Task tool to spawn specialist agents.

Current Behavior

The Task tool accepts a model parameter (sonnet, opus, haiku) but has no way to configure the thinking budget for spawned agents. The ultrathink/megathink/think hard keywords only work in the main CLI session, not when passed in agent prompts.

Proposed Solution

Add an optional thinking parameter to the Task tool / subagent configuration:

# In .claude/agents/*.md
---
name: complex-architect
model: opus
thinking: ultrathink  # or maxThinkingTokens: 31999
---

Or via the Task tool directly:

{
  "subagent_type": "pact-architect",
  "model": "opus",
  "thinking": "ultrathink",
  "prompt": "..."
}

Use Case

When orchestrating multi-agent workflows (like the PACT framework), certain specialist agents handle complex tasks that would benefit from extended thinking:

• Architect agents: Need deep reasoning for system design decisions
• Code review agents: Benefit from thorough analysis of implementation quality
• Test engineers: Could use extended thinking for edge case identification

Currently, there's no way to give these agents more "thinking time" when delegating complex work.

Alternatives Considered

1. Global MAX_THINKING_TOKENS env var - Applies to all requests, no per-agent control
2. Including "ultrathink" in prompt text - Doesn't work; keywords are CLI-level, not parsed from prompts

Additional Context

This would make multi-agent orchestration significantly more powerful for complex software engineering tasks where some agents need deeper reasoning than others.