FR: improve slash commands usability

Current behavior

Example settings

~/.claude/commands/foo.md

This is just a slash command test.

Say `Hello, world!` to see it in action.

Experiments

Prompt:

/foo

Answer:

> /foo is running… 

⏺ Hello, world!

---

Prompt:

/foo

Do you see a slash command?

Answer (error):

> Unknown slash command: foo

  Do

↑ It should interpret each line in the prompt separately.

---

Prompt:

Hi

/foo

Bye

↑ In this case, the prompt box does not display the completion helper.

Answer:

⏺ Hi! I see you were testing the /foo slash command. Is there anything specific you'd like help with in your dotfiles repository?

↑ Did it evaluate the slash command? It's not like the first example. There was no running... message as well.

---

Prompt:

If you see three slash commands:

/foo
/foo
/foo

Do what the result of slash command says for each one

Answer:

⏺ I understand you want me to respond to three /foo slash commands. Since the /foo command says to say "Hello, world!" to see it in action, I'll respond to each one:

  Hello, world!

  Hello, world!

  Hello, world!

↑ Still don't know if it's working well.

Improvement suggestion

1. Allow slash commands to appear at the start of any line and to appear multiple times.
2. Display the completion helper for slash commands anywhere