docs: ADR-0006 Amendment 1 — undo/snapshot design (every-mutation, hybrid, batch) + plan

Settles the undo/snapshot half (U1/U2) before implementation:
- every-mutation single-step undo (supersedes destructive-only model)
- hybrid whole-project snapshot (db backup API + yaml/csv copy),
  reconciling ADR-0006 with ADR-0015's derived-db model
- persisted N=50 ring; redo discarded on new work
- batch ops (replay + future) record one undo step; import excluded
- --no-undo disable switch
Adds the implementation plan and updates README index, requirements
U1/U2, and CLAUDE.md in lockstep.

docs/requirements.md

@@ -365,11 +365,23 @@ handoff-14 cleanup; 449 after B2/C2.)
 
 ## Undo and replay (per ADR-0006)
 
-- [ ] **U1** Auto-snapshot before destructive operations into a
-  ring buffer (initial size N=10, tunable).
-- [ ] **U2** `undo` restores the most recent snapshot; `redo`
-  re-applies; both prompt for confirmation showing the snapshot
-  timestamp and a summary of the changes that will be discarded.
+- [ ] **U1** Auto-snapshot before **every** data/schema mutation
+  (DSL + SQL) into a persisted ring buffer (size N=50, tunable),
+  per ADR-0006 Amendment 1 (single-step undo, superseding the
+  original destructive-only model). Snapshot is a hybrid
+  whole-project copy (database via online backup API + `project.yaml`
+  / `data/*.csv` as files); staged before the mutation's transaction,
+  finalised after the db commit (preserves ADR-0015 §6). A batch
+  command (`replay` / future batch ops) records one boundary
+  snapshot; `import` takes none. A `--no-undo` CLI flag disables
+  snapshotting. *(Designed + amended + planned in
+  `docs/plans/20260524-adr-0006-undo-snapshots.md`; not yet
+  implemented.)*
+- [ ] **U2** `undo` restores the most recent snapshot (database +
+  text, directly); `redo` re-applies (redo stack discarded on new
+  work); both prompt for confirmation naming the command being
+  undone / re-applied (`Y` confirms). *(Designed + amended; not yet
+  implemented — see U1's plan.)*
 - [x] **U3** `history.log` records every submitted command in
   append-only form, tagged with its outcome (Iteration 2;
   broadened by ADR-0034). Format: `<ISO-8601 Z>|<status>|<source>`