/btw fails silently when terminal reports zero dimensions, args leak into conversation

Summary

When running Claude Code in a terminal that reports COLUMNS=0 LINES=0 (specifically cmux v0.61.0, a Ghostty-based terminal), the /btw command fails silently. Instead of showing an error, it falls through to the skill system, which outputs "Unknown skill: btw" and leaks the user's question text into the conversation as "Args from unknown skill:".

Environment

• Claude Code: 2.1.74
• Terminal: cmux 0.61.0 (Ghostty-based, TERM=xterm-ghostty)
• macOS (Darwin 24.6.0)
• /btw works correctly in iTerm2 on the same machine with the same Claude Code version

Steps to Reproduce

1. Open Claude Code in a terminal where COLUMNS=0 LINES=0 (cmux has this bug)
2. Type /btw some question here
3. Observe output: "Unknown skill: btw" and the question text appears as a regular conversation message

Expected Behavior

Claude Code should detect that terminal dimensions are unavailable and show a clear error, e.g.:

/btw requires terminal dimensions to render its overlay. Your terminal is reporting 0x0. Try resizing or using a different terminal.

Actual Behavior

• /btw handler doesn't register (or silently fails)
• Falls through to the skill/slash-command system
• "Unknown skill: btw" is displayed
• The question text leaks into the conversation as "Args from unknown skill:"

Notes

The root cause of COLUMNS=0 LINES=0 is a cmux bug (being reported separately), but Claude Code should handle this gracefully regardless. The silent fallthrough + args leaking into conversation is the Claude Code-side issue.