fix(completion): stop fish from writing debug logs

[jeet-dekivadia]

Summary

• discard stderr from fish's internal completion probe instead of appending it to /tmp/fish-debug.log

Why

The fish completion script currently writes diagnostics from every tab-completion invocation to a fixed temporary file. That leaves an unbounded, cross-session debug log behind during normal CLI usage and can retain command diagnostics unexpectedly. Bash already suppresses the same internal probe's stderr; fish should do the same.

Validation

• git diff --check
• go test ./internal/autocomplete -count=1
• verified the embedded fish completion script no longer references fish-debug

[jeet-dekivadia]

Closing in favor of #21, which now includes this fish stderr fix together with focused regression coverage so reviewers have one cohesive completion-hygiene patch.