cmdValidateHealth read the phases directory four separate times for
checks 6 (naming), 7 (orphaned plans), 7b (validation artifacts), and
8 (roadmap cross-reference). Hoist the directory listing into a single
readdirSync call with a shared Map of per-phase file lists.
Reduces syscalls from ~3N+1 to N+1 where N is the number of phase
directories.
Closes#1973
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>