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

Kanban/Tier/Tier.swift

@@ -23,8 +23,11 @@ public enum Tier: String, Sendable, Equatable, Codable, CaseIterable {
     /// existing at all.
     case free
 
-    /// Lanework Pro — an active auto-renewable subscription. Binds the git history provider when
-    /// pro-m1 builds it (06-history-undo.md, 07-sync-collab.md).
+    /// Lanework Pro — an active auto-renewable subscription. It is what puts git on the table; which
+    /// substrate a given board then binds is the *board's* answer, not this case's (re-ruled
+    /// 2026-07-31 — `AppModel.makeHistoryProvider`): the git provider on a git board
+    /// (06-history-undo.md, 07-sync-collab.md), the same native stack the free tier uses on a gitless
+    /// one, nothing on a repo-nested one.
     case pro
 }