Realign undo with the evening rulings — repo-nested and identity anchors

Repo-nested boards bind native undo in every tier (25d2513): the
no-undo case is gone, makeHistoryProvider answers git or native, and
the native path provably never touches the enclosing repository's
.git. Session undo steps anchor by card identity, never by path
(9119aa1): HistoryAnchor carries the card UUID (plus comment/draft
vocabulary) and apply-time validation resolves the current folder via
the same both-container walk writeCardBody uses — a board-side lane or
trash move no longer stales the coarse close step, while a genuine
field collision still skips it whole.

2448 tests in 423 suites green.

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

Kanban/History/HistoryProviding.swift

@@ -203,10 +203,10 @@ public struct HistoryStep {
 ///
 /// `AppModel.BoardSession` is where the board half's ownership lives, and the composition root binds
 /// which implementation it gets — **following the board, not the tier alone** (re-ruled 2026-07-31):
-/// a gitless board binds `NativeHistoryProvider` (two step stacks over the inverses registered at
-/// the Writer boundary) in every tier, a Pro git board binds the git provider (undo as forward
-/// restore commits over HEAD's first-parent ancestry — 06-history-undo.md), a repo-nested board
-/// binds none at all, and Teams inherits Pro's.
+/// a board without app-managed git — repo-nested included (re-ruled 2026-07-31) — binds
+/// `NativeHistoryProvider` (two step stacks over the inverses registered at the Writer boundary) in
+/// every tier, a Pro git board binds the git provider (undo as forward restore commits over HEAD's
+/// first-parent ancestry — 06-history-undo.md), and Teams inherits Pro's.
 ///
 /// ### What this protocol deliberately does not say
 ///