feat: add undo functionality to revert the last turn and restore file changes

- Implemented `/undo` command to revert the most recent completed turn, restoring all modified files and rewinding message history.
- Introduced `FileMutation` type to capture file changes for undo operations.
- Enhanced session management to track and manage undo stack with a maximum of 20 turns.
- Updated command parsing to include user-defined commands from `.shiro/commands.md`, allowing for custom command definitions and substitutions.
- Added documentation for the new `/undo` command and user commands feature.
- Implemented tests for undo functionality and user command parsing to ensure reliability.

test/helpers.ts

@@ -33,6 +33,10 @@ export function testHooks(over: Partial<AppHooks> = {}): AppHooks {
     initPrompt: 'write AGENTS.md',
     history: [],
     recordPrompt: () => {},
+    listUserCommands: () => [],
+    expandUserCommand: async (name) => {
+      throw new Error(`no custom command "${name}" in tests`);
+    },
     ...over,
   };
 }