WL-0MLGBAPEO1QGMTGM: Migrate GitHub throttling to token-bucket at helper level

[SorraTheOrc]

Summary

This PR centralizes GitHub API throttling using a TokenBucketThrottler and moves scheduling to the helper level to avoid accidental serialization of internal parallel operations. Key points:

• Adds (configurable via , , ) with debug tracing (enabled with or ).
• Migrates and to schedule the entire helper as a single throttled task instead of per-call scheduling, preserving internal concurrency.
• Removes several per-call usages to reduce unintended serialization.
• Adds unit tests for throttler and verified the two timing-sensitive CLI tests.

Testing

• Ran timing-sensitive tests with throttler debug enabled:
  • (pass)
  • (pass)
• Ran full test suite: all tests pass.

Notes

• The throttler does not change existing retry/backoff/secondary-rate-limit logic implemented in the helpers.
• If desired we can further audit remaining code paths for scheduling granularity and consider a grouped reservation API later.

/cc @team