Add an interactive welcome and workflow picker to `/workflow`

[pmateusz]

Description

The bare /workflow command should open a recognizable entry screen for Kimchi Workflows instead of returning only a plain notification. The screen should confirm that the command was received, explain what workflows are, and provide an immediate next action based on whether the project already contains workflows.

Today, /workflow behaves like /workflow list:

• when no workflows exist, it prints a single message with the workflows directory and tells the user to run /workflow create;
• when workflows exist, it prints a text list, after which the user must type a separate /workflow run <name> command.

This is functional but provides little onboarding and makes the most common actions harder to discover than necessary.

Desired experience

Project with no workflows

The screen should:

• show a clear Kimchi Workflows header so the user knows the command was accepted;
• include a short subtitle explaining the feature;
• state that the project has no workflows yet;
• show where workflow files are stored so users can also add or import one manually;
• present Create a workflow as the primary action, selectable with Enter when it is the only available action.

Project with existing workflows

The screen should:

• list available workflows with enough context to identify them;
• allow the user to select and run a workflow without typing another command;
• keep Create a workflow available as a secondary action;
• preserve command-based operation for users who prefer /workflow run <name>.

Expected outcome

Running /workflow gives new users clear onboarding and gives returning users a fast workflow picker. The command should feel like the entry point to a feature rather than a plain list response.