[Feature Request] Add thinking frontmatter option for slash commands

Bug Description
Title:
Add thinking frontmatter option for slash commands

Description:
It would be useful to control extended thinking mode at the slash command level via frontmatter.
Currently, thinking is only controllable at the session level (Tab key toggle or alwaysThinkingEnabled setting).

Use case: Commands like /commit that use faster models (e.g., claude-haiku-4-5-20251001) don't benefit as much from extended thinking and would run even faster without it.
Users with alwaysThinkingEnabled: true have no way to disable thinking for specific quick commands.

Proposed solution:

---
model: claude-haiku-4-5-20251001
thinking: false
---

This would override the session/global thinking setting for that specific command.

Environment Info

• Platform: win32
• Terminal: mingw64
• Version: 2.0.58
• Feedback ID: 6ebc24e2-d575-474e-9d85-659196e62d2f