The v1.29.0.0 changelog promised .gbrain-source would be added to the
consuming repo's .gitignore so the per-worktree pin stays local, but the
change actually only added it to gstack's own .gitignore. Without the
consumer-side entry, the pin gets committed and Conductor sibling
worktrees of the same repo + branch step on each other's pin every time
anyone commits.
Add ensureGbrainSourceGitignored after a successful gbrain sources
attach in runCodeImport. Idempotent on repeat runs (line-trim match),
creates .gitignore if missing, logs a warning and continues on
permission errors so a read-only checkout doesn't fail the sync.
Gate the top-level main() call behind import.meta.main so tests can
import the helper without triggering a full sync run on module load.
Tests in test/gbrain-source-gitignore.test.ts cover: create-when-missing,
append-without-trailing-newline, append-with-trailing-newline,
idempotent on repeat, recognize whitespace-surrounded entry, no-throw
on read-only file. 6 pass.