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.

CLAUDE.md

@@ -58,9 +58,14 @@ Current decisions at a glance (each backed by an ADR):
   supported. No real UUIDs (ADR-0005). FK column type
   compatibility via `Type::fk_target_type()` — `serial → int`,
   `shortid → text`, others identity (ADR-0011).
-- **Safety:** auto-snapshot before destructive ops; `:undo`;
-  append-only `history.log` for replay and scripting (ADR-0006).
-  *(Designed; not yet implemented.)*
+- **Safety:** append-only `history.log` for replay and scripting
+  (ADR-0006 U3/U4) — *implemented* (ADR-0034). Undo/snapshot half
+  (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.)*
 - **Sharing:** `export` command produces a zip without the `.db`;
   no hosted publishing (ADR-0007).
 - **Testing:** four-tier strategy from `cargo test` units up to