fix(tui): allow slash command menu on first line with existing text

[aliou]

Hello!

Noticed that when trying to prepend a custom command when the editor has multiple line, the slash command completion wouldn't trigger.

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Summary by Opus:

Changes isSlashMenuAllowed() in packages/tui/src/components/editor.ts from checking that all non-cursor lines are empty to simply checking cursorLine === 0.

Before: Typing / at the beginning of line 0 would not trigger the slash command autocomplete menu if any other line had content. This prevented the workflow of writing text first, then prepending a slash command to use that text as the command's argument.

After: The slash command menu triggers on line 0 regardless of content on other lines. It still does not trigger on non-first lines, preserving the fix for #904 (slash menu appearing on arbitrary new lines in multi-line input).

File: packages/tui/src/components/editor.ts

• isSlashMenuAllowed(): changed from a loop checking all lines to return this.state.cursorLine === 0
• isAtStartOfMessage() and isInSlashCommandContext() are unchanged but inherit the new behavior

Commands that already accept arguments via startsWith (/model, /compact, /name, /export) work with trailing text. Commands that use exact match (/settings, /new, etc.) are unaffected since they don't accept arguments.

[badlogic]

That's on purpose. What's the use case for this?

[aliou]

I use a few commands (like handoff or skill) but sometimes forget to start my message. So I go back to the start of the editor to add it, but the completion doesn't trigger I often don't remember from the top of my head the exact name of the command/skill I need to use, just enough for the tab to work.

Right now i end up copying the text, clearing the editor, typing+tab the command and then re-pasting the rest of the message.

[badlogic]

Looking good, merged!