v0.7.2: undo/redo — TDD

Operation-level memory undo/redo built on existing Merkle snapshot
infrastructure. undo-snapshot captures state before destructive tool
execution. /undo and /redo TUI commands send structured events.

- core-memory: undo-snapshot, undo, redo functions + 3 tests
- core-perceive: :undo/:redo sensor handlers
- core-act: auto-snapshot before non-read-only tools
- core-package: undo/redo symbol exports
- channel-tui-main: /undo, /redo commands + 2 tests
- Core: 73/73  TUI Main: 74/74

lisp/core-act.lisp

@@ -81,6 +81,9 @@
          (meta (getf context :meta))
          (source (getf meta :source))
          (tool (gethash (string-downcase (string tool-name)) *cognitive-tool-registry*)))
+    ;; v0.7.2: snapshot before destructive tool execution
+    (when (and tool (not (cognitive-tool-read-only-p tool)))
+      (undo-snapshot))
     (if tool
         (handler-case
             (let* ((clean-args (if (and (listp tool-args) (listp (car tool-args))) (car tool-args) tool-args))