feat(push): parallelize push operations with goroutines

[jamesjohnsdev]

Serial push was a bottleneck when many issues needed syncing.
Both (all-modified) and (new T-issues) now fan out
up to 10 concurrent gh API calls. Filter pass stays serial to avoid
concurrent isModified disk-read races.

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Summary by cubic

Parallelize push operations to speed up syncing when many issues need updates. Modified and new T-numbered issues now push with up to 10 concurrent GitHub API calls, while disk-read filtering stays serial to avoid races.

• New Features
  • push: Parallel pushes for modified issues (10-worker semaphore), with an atomic count and “Nothing to push” when zero.
  • sync: Parallel pushes for new T-numbered issues before reload, with a summary count.
  • Error handling: Collects worker errors and returns the first encountered.

^{Written for commit 9f9531d. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}