Bind the undo provider to the board, not the tier

The 2026-07-31 re-ruling: gitless boards bind the native stack in every
tier — a Pro upgrade no longer removes undo from mode-none boards — and
Pro git boards bind the git provider; repo-nested stays the no-undo
case under Pro, while the free tier (which never runs detection) binds
native there too, per 12's inert posture. Add-git now swaps a live
native substrate mid-session: the in-flight stack is cleared with the
discarded provider, the git trail seeds from the root commit, and the
same BoardUndoManager instance keeps nil-target menu validation fresh.

2405 tests in 413 suites green.

Claude-Session: https://claude.ai/code/session_01CqjXB7ASoWtbyoGod68k97

Kanban/Tier/ProSettingsSection.swift

@@ -29,9 +29,10 @@ import SwiftUI
 ///
 /// "Subscribe takes effect at each board's next open ... The purchase flow offers to reopen open
 /// boards so the upgrade feels immediate" (12 ▸ The entitlement). The offer is raised **once**, from
-/// here, on the one outcome that means an active subscription just landed — and declining costs
-/// nothing, today least of all: both tiers bind the native history stack until pro-m1 builds the git
-/// provider (`AppModel.makeHistoryProvider`).
+/// here, on the one outcome that means an active subscription just landed — and declining costs a
+/// gitless board nothing at all: the provider follows the board, so its undo is the same native
+/// stack before and after (`AppModel.makeHistoryProvider`). What a Not Now defers is the git trail
+/// on the boards that have a repository.
 struct ProSettingsSection: View {
 
     @Environment(AppModel.self) private var appModel