Allow command templates to be invoked via slash commands

### Problem

Command templates can be created, but they cannot currently be invoked directly from the chat input using slash-command syntax.

This makes reusable prompts less discoverable and less efficient to use, since users need to manually locate and insert the template instead of calling it by name.

### Expected behavior

A command template should be available as a slash command, similar to pi.dev prompt templates.

For example, a template named `review-pr` should be invokable with:

/review-pr

Typing `/` should ideally provide autocomplete for available templates, including their name and description.

### Actual behavior

Command templates cannot currently be invoked through the slash-command interface.

### Example use cases

- `/review-pr` to start a standardized pull-request review
- `/create-issue` to generate an issue using a team-specific template
- `/architecture-review ` to apply a reusable architecture-review prompt

### Questions

- Is slash-command support for command templates intentionally not supported?
- Is there an existing workaround or configuration that enables it?
- Should template arguments be supported, e.g. `/review-pr 123`?
- Should templates be scoped globally, per user, per workspace, and/or per project?

Start by tracing the chat-input slash-command entry point and the existing command-template flow, then inspect how template names, descriptions, arguments, and scopes are represented. Done means a template such as /review-pr can be discovered through slash autocomplete and invoked from chat, with argument and scope behavior decided and verified.

Written by the indexing model from the issue text.