Provider management: remove a connected provider from the TUI #1294

Problem

An operator who connects a provider account (e.g. xai/default-2) has no in-product way to remove it again. Today the only path is hand-editing global settings.json plus the OAuth credential files, which easily leaves defaultProvider / recentModels / favorites pointing at a deleted entry or an orphaned auth profile projecting a ghost row into /model.

Desired

• Row action in the /model picker to delete the focused provider account, with a two-step armed confirm showing the blast radius.
• Deletes the catalog row plus the credential (OAuth profile via AuthStore.removeProfile, API-key secret in settings); keyless entries touch settings only.
• Deterministic reference repair (recentModels, favorites, defaultProvider repoint) and a live-session guard that refuses to delete the provider the session is running on.
• Notice summarizing what was removed and what default was chosen.

Plan

Scratch plan at docs/plans/provider-remove/ (PRD, SOLUTION_SCOPE, BUILD_PLAN, planning-only, gitignored). Durable conclusions to land in docs on implementation.