feat: add T (select thinking) and h (hide thinking) to transient menu

[DevGiuDev]

• T: select thinking level from list via completing-read (off, minimal, low, medium, high, xhigh) using set_thinking_level RPC
• h: toggle hide-thinking — hides streaming thinking blocks client-side, shows placeholder 'Thinking...' label instead
• Header-line and transient description show [hidden] when active
• t renamed to 'thinking (cycle)', T to 'Thinking (list)'

[dnouri]

I implemented the feature where you can select the thinking level in #191. It replaces the old t binding because that's no longer useful (however you can still cycle by using the mouse), and because T was already taken.

As for the hide thinking, I like the idea, but I'd like the implementation to be more like a real hide with the ability to toggle hiding with true round-trip and without loss of thinking text, maybe a bit similar to how we're toggling long outputs today.

[DevGiuDev]

You mean to have thinking collapsed? This is better than my solution of course

[dnouri]

You mean to have thinking collapsed? This is better than my solution of course

Yes, exactly.

[dnouri]

@DevGiuDev Do you wanna give #195 a try? It implements the collapsable "hide thinking".

[DevGiuDev]

Ok, will try it later.