[UX]: Toggle between Chat and Shell modes with ! and @ commands

### Checks

- [x] I have searched [github.com/aws/amazon-q-developer-cli/issues](https://github.com/aws/amazon-q-developer-cli/issues?q=) and there are no duplicates of my issue
- [x] I have run `q doctor` in the affected terminal session
- [x] I have run `q restart` and replicated the issue again

### Operating system

macOS 15.3.2 (24D81)

### Expected behaviour

To allow for complex series of self-type shell commands, without needing to exit back to the zsh shell...

Users should be able to:
1. Type "!" on a line by itself to switch from chat mode (>) to shell mode ($)
2. Execute multiple shell commands consecutively in shell mode
3. Type "@" on a line by itself to switch back to chat mode
4. Maintain context and history between mode switches

### Actual behaviour

Currently, users need to prefix each shell command with "!" or use execute_bash for running commands, which is less efficient for running multiple consecutive commands.

### Steps to reproduce

This is a feature request for new functionality that doesn't exist yet in the current q chat CLI.

### Environment

```yaml

```