[FEATURE] Selecting slash command from autocomplete should insert text, not execute immediately

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 typing a slash command like /my-c, the autocomplete dropdown appears with matching commands (e.g., /my-command). Selecting a command from this dropdown immediately executes it without giving the user a chance to append arguments.

This is problematic for any custom command that accepts arguments. For example:

• I want to run /my-command some-argument
• I type /my-c and select /my-command from the dropdown
• The command executes immediately with no arguments
• I have to dismiss the output and retype the full command with arguments

This makes the autocomplete actively counterproductive for commands with arguments — it's faster to ignore the dropdown entirely and keep typing the full command manually.

Proposed Solution

When a user selects a slash command from the autocomplete dropdown, insert the command text into the input field instead of executing it immediately. This lets the user append arguments before pressing Enter to submit.

Expected behavior:

1. User types /my-c
2. Dropdown shows /my-command
3. User selects /my-command
4. Input field now contains /my-command (with trailing space, cursor at end)
5. User types their arguments and presses Enter
6. Command executes with arguments

This matches autocomplete behavior in terminals, IDEs, and other CLI tools — autocomplete completes text, it doesn't submit it.

Alternative Solutions

• Current workaround: Ignore autocomplete entirely and type the full command with arguments manually. Works but defeats the purpose of autocomplete.
• Commands can detect empty $ARGUMENTS and display help text, but it adds an extra round-trip.
• A configuration option to toggle between "insert" and "execute" behavior on autocomplete selection.

Priority

Medium - Would be very helpful

Feature Category

Interactive mode (TUI)

Use Case Example

Any custom slash command that accepts arguments hits this:

1. I type /dep intending to run /deploy staging
2. I see the autocomplete dropdown and select /deploy
3. The command runs immediately with no arguments — deploys to the wrong environment or errors out
4. I have to retype /deploy staging from scratch

The longer the command name, the worse it gets — autocomplete should save keystrokes on long command names, but instead it prevents you from entering arguments at all.

Additional Context

This likely affects a large number of users since custom slash commands with $ARGUMENTS are a common pattern. The current behavior trains users to avoid the autocomplete dropdown, which is the opposite of its purpose.