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/Git/GitAutoCommitter.swift

@@ -161,8 +161,8 @@ public final class GitAutoCommitter {
     ///
     /// `nil` wherever no `GitHistoryProvider` is listening — which in practice is nowhere a committer
     /// exists at all: a committer's existence is exactly mode `git`, and mode `git` is exactly where
-    /// the composition root binds the git provider (`AppModel.makeHistoryProvider`). Mode-none boards
-    /// have a native stack and no committer; repo-nested boards have neither.
+    /// the composition root binds the git provider (`AppModel.makeHistoryProvider`). Mode-none and
+    /// repo-nested boards alike have a native stack and no committer.
     @ObservationIgnored
     public var reportLanded: (@MainActor (GitLandedWindow) -> Void)?