Slash command autocomplete should trigger anywhere in the prompt, not just at the start

Feature Request

Current behavior: The / slash command autocomplete dropdown only appears when / is typed at the very beginning of the prompt input. If you type some text first and then type /, no autocomplete appears.

Desired behavior: The slash command autocomplete should trigger whenever / is typed, regardless of position in the prompt — similar to how Cursor handles / commands in their Composer input.

Why this matters

Projects with many custom commands (we have 145+) rely heavily on slash commands for agent orchestration, team coordination, and workflow automation. Common usage patterns include:

/gran go ahead and /review-code the latest changes
/family what's the /progress on WCR
Please /analyze this and then /save it

The commands still execute correctly when typed mid-prompt — it's only the autocomplete/intellisense that's missing. This forces users to memorize exact command names instead of browsing the dropdown.

Context

• Claude Code CLI (terminal) + Desktop app
• Custom commands defined in .claude/commands/
• Heavy command usage in multi-agent orchestration workflows
• Cursor already supports this pattern — users switching between tools expect the same behavior

Suggested Implementation

Detect / followed by word characters at any cursor position in the input, not just ^/. Show the same autocomplete dropdown that currently appears at position 0.