fix: remove trailing dash from issue-prefix in beads config

ensure_config_yaml wrote issue-prefix with a trailing dash (e.g. "hw-"),
causing bd to generate IDs with a double-dash prefix ("hw--"). bd adds
its own separator, so the config value should be just the prefix.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: julianknutsen

cmd/gc/gc-beads-bd

@@ -777,7 +777,7 @@ ensure_config_yaml() {
     tmp=$(mktemp "$config_file.tmp.XXXXXX")
     cat > "$tmp" <<YAML
 issue_prefix: $prefix
-issue-prefix: ${prefix}-
+issue-prefix: ${prefix}
 dolt.auto-start: false
 YAML
     mv "$tmp" "$config_file"