auth: remove contexts under a single locked Modify

Review finding: the logout-failsafe rework split context removal into a Load and a separate Modify, reopening the read/write lock gap the contexts package warns about — a concurrent auth use in the window could retarget which context "current" means mid-logout. Selection, keyring deletion, and entry removal now all run inside one locked Modify; a failed credential delete aborts the callback, which discards the entry change and preserves the retry-friendly failure mode.