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.

docs/user-commands.md

@@ -0,0 +1,36 @@
+# User commands
+
+Define your own slash commands in a markdown file: `.shiro/commands.md` at the
+workspace root. Each command is a `## <name>` heading; an optional `> summary`
+line under it becomes the `/`-menu description; the body is the prompt template
+sent to the model.
+
+```markdown
+# My commands
+
+## review
+> review a change against the project rules
+
+Read @AGENTS.md first, then critique:
+$ARGUMENTS
+
+## scaffold
+> make a new module
+
+Create a module named $1 in src/, with a test.
+```
+
+The body supports these substitutions:
+
+- `$ARGUMENTS` — everything typed after the command name, verbatim.
+- `$1` .. `$9` — the nth whitespace-separated argument (empty when absent).
+- `` !`cmd` `` — replaced with the trimmed stdout of running `cmd` in a shell.
+- `@path` — replaced with the contents of the file at `path` (workspace-rooted).
+
+Substitutions apply in a safe order: shell reads first, then files, then `$n`
+tags, so text a shell call produces is not itself re-read. A missing file or
+failing shell keeps its literal text plus a bracketed note rather than throwing.
+
+A user command whose name collides with a built-in is shadowed — the built-in
+wins — so a project cannot hijack `/model` or `/help`. Type `/name` to run one;
+it is also listed in the `/` completion menu.