These files are always sourced (never executed directly), so the
#!/bin/bash shebang is inert and misleading. Drop it consistently
across rc.d/*, profile.d/*, and aliases.d/*.
Merge rc.d/mcfly and rc.d/atuin into rc.d/history, following the same
preference-list + availability-check + fallback pattern as rc.d/prompt,
so only one history tool is active at a time instead of both being
initialized independently. Also normalize variable expansions in
rc.d/prompt to consistently use ${var} braces.
xavier