[FEATURE] Slash command autocomplete should insert command text without immediately executing

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When selecting a slash command from the autocomplete menu (via Tab or Enter), the command is immediately executed. There is no way to select a command and then append arguments or text before submitting.

This is particularly frustrating for commands that accept arguments (e.g., /loop, /review, custom commands with $ARGUMENTS). The user has to either:

• Type the full command name manually without using autocomplete
• Execute the command without arguments and lose the opportunity to pass them inline

Proposed Solution

When a slash command is selected from the autocomplete menu, insert the command text into the input field instead of executing it immediately. This would allow the user to:

1. Use autocomplete to select the command
2. Add arguments or additional text
3. Press Enter to execute when ready

This is consistent with how most shells and IDEs handle autocomplete — selection fills the input, submission is a separate action.

Priority

Medium

Feature Category

CLI commands and flags

Use Case Example

Current behavior:

1. Type /loo
2. Autocomplete suggests /loop
3. Press Tab or Enter to select
4. /loop executes immediately with no arguments

Expected behavior:

1. Type /loo
2. Autocomplete suggests /loop
3. Press Tab or Enter to select
4. Input field now contains /loop (with cursor ready)
5. Type 5m /review
6. Press Enter to execute /loop 5m /review

Additional Context

• Related to #11431 which also addresses slash command autocomplete UX issues
• This affects all slash commands that accept arguments