Persistent, optional heads-up notes from Pi extensions

[aliceisjustplaying]

Would T3 accept a small, persistent surface for optional Pi extension notes above the composer?

The concrete use case is pi-you-should-know, which supplies background notes and explanations. My fork displays those notes with Explain, Knew, Ask agent, Dismiss and Undo. Knew records that the user already knows the information. Feedback goes back to the originating Pi session, including after T3 has released its process.

The fork implementation is a concrete proposal, not a request to accept its protocol unchanged. It currently recognizes the extension's [ysk:id] format and implements the band for web/desktop; mobile has no equivalent UI yet.

An integration smoke check passed with real Pi and isolated state: receive an idle note, persist it in T3, release Pi, resume for Knew feedback, verify Pi's saved knowledge and Undo the acknowledgment. This check makes no model call.

Before opening a feature PR, I'd like agreement on whether this surface belongs in T3, whether the extension-specific format is acceptable or a shared contract is needed and whether web/desktop is an acceptable first scope. The Pi wake and idle-reaper fixes discussed in #15589 are separate reliability work.

Prepared with GPT-6 in Codex.