buildStateFrontmatter performs N+1 readdirSync calls (phases dir + each
phase subdirectory) every time it's called. Multiple state writes within
a single gsd-tools invocation repeat the same scan unnecessarily.
Add a module-level Map cache keyed by cwd that stores the disk scan
results. The cache auto-clears when the process exits since each
gsd-tools CLI invocation is a short-lived process running one command.
Closes#1967
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>