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/usercommands.test.ts

@@ -0,0 +1,46 @@
+import { expect, test } from 'bun:test';
+import { expandCommand, parseUserCommands } from '../src/usercommands';
+
+test('parses name, summary, and body from markdown headings', () => {
+  const commands = parseUserCommands(`# scratch
+
+## review
+> summarise this change
+
+Summarise the diff. Be concise.
+
+## scaffold
+> make a new module
+
+Create a module called $1.
+`);
+  expect(commands).toHaveLength(2);
+  expect(commands[0]!.name).toBe('review');
+  expect(commands[0]!.summary).toBe('summarise this change');
+  expect(commands[0]!.body).toContain('Summarise the diff. Be concise.');
+  expect(commands[1]!.name).toBe('scaffold');
+});
+
+test('lowercases command names so /Review matches review', () => {
+  const commands = parseUserCommands('## Template\nbody');
+  expect(commands[0]!.name).toBe('template');
+});
+
+test('substitutes $1 and $ARGUMENTS', async () => {
+  const [cmd] = parseUserCommands('## x\nUse $1 heavily and all of $ARGUMENTS.');
+  const out = await expandCommand(cmd!, 'foo bar baz', process.cwd());
+  expect(out).toBe('Use foo heavily and all of foo bar baz.');
+});
+
+test('reads @path files relative to the workspace root', async () => {
+  const [cmd] = parseUserCommands('## r\nRead here:\n@package.json\nThen continue.');
+  const out = await expandCommand(cmd!, '', process.cwd());
+  const content = (await Bun.file('package.json').text()).trimEnd();
+  expect(out).toContain('Read here:\n'.concat(content).concat('\nThen continue.'));
+});
+
+test('expands !shell output inline', async () => {
+  const [cmd] = parseUserCommands('## s\nBranch is !`git branch --show-current`.');
+  const out = await expandCommand(cmd!, '', process.cwd());
+  expect(out).toMatch(/Branch is (main|[a-z-]+)\./);
+});