Extend confirm_close to tab close (or add a per-scope setting)

[bjrb20]

Setting confirm_close = true prompts before closing a workspace, but closing a tab (close_tab, default prefix+shift+x) is always silent. A tab often carries the same thing the workspace confirmation protects — a running agent or long-lived process — so losing one to a stray close chord hurts just as much, and there is currently no config option that covers it.

It would be great if either:

• confirm_close = true also applied to close_tab (and possibly close_pane), or
• confirmation were configurable per scope, e.g. confirm_close = ["workspace", "tab"].

Encountered on Herdr 0.8.2 (stable) on macOS, but the behavior is the same per the config docs, which describe confirm_close as workspace-only.

Related prior threads: #1589 asked for this as an issue and was closed by policy (issues are for reproducible bugs); #1750 is an open bug for the narrower case where closing the last tab tears down the whole workspace with no confirmation. This idea is the general form: an opt-in confirmation for any tab close.