Undo reported "nothing left to undo" after a successful undo

Found by running the real thing: the agent replaced a bed of garlic with
cucumbers, Undo restored the garlic correctly — and then said "Nothing left to
undo — this was already reversed."

The revert response carries the change set commitScope just inserted, and that
row has no per-op tally: Counts is populated by the history LIST query, not by
the insert. describeUndo treated a zero tally as "nothing happened", so a real
undo with eight revisions behind it reported itself as a no-op. Telling someone
an action did nothing when it just worked is about the worst thing a safety
feature can say, because the obvious next move is to do it again.

Fixed on both sides. The client now treats a NULL change set as the no-op
signal, which is what the server actually means by it — an empty tally is not
the same thing and must not be read as one. And RevertChangeSet now fills in the
counts, so the revert response and the list read describe the same change set
the same way, and a partial revert can say "2 of 3" from either.

The unit test missed it because the fixture I wrote populated counts, so it was
testing my assumption about the response rather than its actual shape. The new
tests pin the real thing: the service asserts a revert's counts match the fill
it undid AND match what the list read reports, and the client asserts an empty
tally still reads as "Undone."

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01H3zbym8Doka2d7D48maSgZ

web/src/lib/history.test.ts

@@ -49,6 +49,14 @@ describe('describeConflict', () => {
 describe('describeUndo', () => {
   const target = changeSet({ counts: [{ entityType: 'object', op: 'update', n: 3 }] })
 
+  // The bug this pair exists for: a real revert response carries a change set
+  // whose counts the server didn't populate. Reading that as "nothing happened"
+  // told the user a successful undo had done nothing.
+  it('trusts the change set, not the tally, for whether anything happened', () => {
+    const out = describeUndo({ id: 5 }, { changeSet: changeSet({ id: 6, counts: [] }), conflicts: [] })
+    expect(out).toEqual({ tone: 'ok', message: 'Undone.' })
+  })
+
   it('is a plain success when nothing conflicted', () => {
     const out = describeUndo(target, {
       changeSet: changeSet({ id: 2, counts: [{ entityType: 'object', op: 'update', n: 3 }] }),
@@ -61,6 +69,7 @@ describe('describeUndo', () => {
   // already a no-op — reachable by undoing a creation whose object is gone.
   // Claiming "Undone." there reports work that didn't happen.
   it("doesn't claim to have undone a no-op", () => {
+    // A NULL change set — not an empty tally — is the server's no-op signal.
     const out = describeUndo(target, { changeSet: null, conflicts: [] })
     expect(out.tone).toBe('ok')
     expect(out.message).toBe('Nothing left to undo — this was already reversed.')