Fix GitHub search rate throttling

[rogerchappel]

Summary\n- scale REST throttling buffer to the actual GitHub bucket limit instead of assuming the 5k core REST bucket\n- avoid immediate sleep loops for GitHub search buckets, which are often around 30/min\n- serialize per-user commit/PR search calls so they do not race the shared throttle state\n\n## Why\nThe live refresh was sleeping immediately with repeated:\n\n\n[throttle] Approaching rest rate limit — sleeping 60s until reset.\n\n\nThat happened because GitHub search exposes a much smaller REST-style bucket, and the old hard-coded 100-request REST buffer is larger than the whole search bucket.\n\n## Verification\n- pnpm run check\n- pnpm run test