fix(completion): hide internal commands and suppress fish debug logs

[jeet-dekivadia]

Summary

• filter hidden commands from shell completion when no argument has been typed yet
• discard stderr from fish's internal completion probe instead of appending it to /tmp/fish-debug.log
• add focused regressions for the empty-input filter and embedded fish script

Why

The non-empty completion path already skips child.Hidden, but the initial empty-input branch did not. An initial tab press could therefore expose internal commands such as __complete, @completion, and @manpages.

The fish completion script also appended diagnostics from every tab-completion invocation to a fixed temporary file. Bash already suppresses the same internal probe's stderr. Applying the same behavior in fish avoids an unbounded cross-session debug log during normal CLI use.

Validation

• git diff --check
• go test ./internal/autocomplete -count=1

Remote Linux validation

• git diff --check
• ./scripts/bootstrap
• ./scripts/lint
• ./scripts/test

[jeet-dekivadia]

Small follow-up for review context. This patch is limited to completion behavior: hidden commands stay hidden at the shell-completion boundary, and the fish probe no longer writes debug noise to /tmp/fish-debug.log. The regressions cover both the hidden-command filtering and the fish script stderr behavior without touching command execution itself.