Pause session: park a thread's provider to save RAM without settling

[OmarAlaaeldein]

Problem

Snoozing a thread only hides it from the inbox; the provider session keeps running with the same footprint. Settling frees the session but means "done". There is no middle state for parking work, which hurts on RAM-constrained devices.

Proposal

A Pause session thread action: stop the provider subprocess while the thread stays active. Resume is sending the next message — the server already recreates a stopped session on turn start (ensureSessionForThread treats stopped as absent), so no protocol change is needed. Pause is blocked mid-turn like archive (interrupt first), and paused threads show a muted Paused indicator on web and mobile.

Reference implementation

Opened as PR #12172 to make the proposal concrete and reviewable. Happy to shrink, reshape, or drop it based on maintainer direction.