Add command-specific help to `/help` command

## Description
Currently, the `/help` command only shows general help information for all commands. It would be useful to add support for showing help for specific commands, such as `/help context` or `/help profile`.

## Current Behavior
When a user types `/help`, they see a general overview of all available commands.
When a user types `/help `, it still shows the general help instead of specific help for that command.

## Desired Behavior
- `/help` should continue to show general help information for all commands
- `/help ` should show detailed help information for the specified command
- For example, `/help context` should show detailed help for the context command and its subcommands

## Benefits
- Users can get more detailed help for specific commands without having to remember all the different help commands
- Consistent with how help commands work in other CLI tools
- Reduces the need for separate help subcommands for each command

## Examples

│ /help
[Shows general help for all commands]

│ /help context
[Shows detailed help for the context command]

│ /help profile
[Shows detailed help for the profile command]

│ /help nonexistent
Command 'nonexistent' not found. Here's the general help:
[Shows general help for all commands]

## Related Work
- This feature would complement the existing command registry system implemented as part of RFC 0002: internal_command_tool
- Related to the command registry migration plan that ensures consistent behavior between direct command execution and tool-based execution

🤖 Assisted by [Amazon Q Developer](https://aws.amazon.com/q/developer)