[FEATURE] Support tab completion for command arguments (e.g. /effort low/medium/high)

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

Currently, Claude Code supports tab completion for commands (e.g. /eff → /effort),
but not for command arguments.

It would be helpful to have tab completion for argument values like:
/effort [tab] → shows: low / medium / high

This would improve usability especially for frequently used commands.

Proposed Solution

When the user presses Tab after a command that accepts a predefined set of arguments,
Claude Code should display and cycle through the available options, similar to how
shell completion works.

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

CLI commands and flags

Use Case Example

_No response_

Additional Context

_No response_