docs: ADR-0006 §8 step 8 — mark undo/snapshot implemented

- requirements.md U1/U2: [ ] -> [x] with implementation notes
- CLAUDE.md Safety bullet: implemented; drop U-series from the
  deliberately-deferred list
- ADR-0006 Amendment 1: "not yet implemented" -> implemented, plus an
  Implementation note (index.yaml, source-gated snapshots, non-fatal
  snapshot-failure policy, batch primitive, testing + accepted gaps)
- ADR README index: undo/snapshot half marked implemented

CLAUDE.md

@@ -63,9 +63,9 @@ Current decisions at a glance (each backed by an ADR):
   (U1/U2): `undo` / `redo` app commands (no sigil) with auto-snapshot
   before **every** mutation into a persisted N=50 ring; hybrid
   whole-project snapshot (db backup API + yaml/csv copy); `--no-undo`
-  to disable (ADR-0006 **Amendment 1**). *(Designed + amended +
-  planned in `docs/plans/20260524-adr-0006-undo-snapshots.md`; not
-  yet implemented.)*
+  to disable (ADR-0006 **Amendment 1**). *(Implemented 2026-05-24 —
+  `src/undo.rs` ring + worker hook in `src/db.rs`; one undo step per
+  user command, batch ops collapse to one, `import` excluded.)*
 - **Sharing:** `export` command produces a zip without the `.db`;
   no hosted publishing (ADR-0007).
 - **Testing:** four-tier strategy from `cargo test` units up to
@@ -203,8 +203,6 @@ not yet implemented:
 - **Strong syntax-help in parse errors** (H1a): point users at
   missing keywords/clauses rather than the unexpected
   character.
-- **Snapshot/replay/undo** (U-series): designed in ADR-0006;
-  not yet implemented.
 - **Tutorial/lesson system**: acknowledged as in scope for
   design; needs its own ADR.
 - **Session log + Markdown export** (V4): the bigger UX