[BUG] Slash command autocomplete ignores exact matches and executes wrong command

Preflight Checklist

• [x] I have searched existing issues to ensure this bug hasn't already been reported
• [x] This is a single bug report, not multiple issues
• [x] I am using the latest version of Claude Code

## What's Wrong?

The slash command autocomplete ignores exact matches and prioritizes fuzzy/substring
matching instead. When I type /clear completely, the autocomplete still shows
/commit first and pressing Enter executes /commit instead of /clear.

This behavior did not occur earlier when I had fewer slash commands installed.
The bug appears to worsen or only manifest when many custom commands/skills are
available (~31 total in my setup).

## What Should Happen?

The autocomplete ranking should prioritize:

1. Exact matches (highest priority) - /clear typed fully should match /clear
2. Prefix matches (high priority) - /cle should match /clear before /commit
3. Substring/fuzzy matches (lower priority)

When I press Enter, the command that is shown/highlighted should execute, or if I've
typed a complete exact match, that command should execute.

## Steps to Reproduce

1. In the Claude Code CLI, start typing a slash command: /cle
2. Observe that /commit appears first in autocomplete (not /clear)
3. Continue typing the full command: /clear
4. /commit still appears first in suggestions (not /clear)
5. Press Enter expecting /clear to execute
6. /commit executes instead

## Claude Code Version

2.1.19

## Platform

macOS (Darwin 24.4.0)

## Additional Information

Environment:

• Shell: zsh
• Installed skills/commands: 31 custom skills (15 user skills + 16 plugin skills)

Regression pattern:

• Worked correctly with fewer slash commands installed
• Broke after reaching ~31 total custom commands
• Suggests autocomplete algorithm degrades with larger command sets

Impact:
This is dangerous - users type one command, see another suggested, and have a third
command execute. Particularly problematic for destructive commands.

Workaround:
Use arrow keys to manually select the correct command instead of pressing Enter on
the first suggestion.

Potential root cause:
May be related to performance optimization that skips proper ranking when command
list exceeds a threshold (possibly SLASH_COMMAND_TOOL_CHAR_BUDGET).