feat: add adaptive patch compression for large PRs (#30)

4-stage progressive degradation strategy for diffs exceeding token budget:
  Stage 1 (Full): all diffs fit → use as-is
  Stage 2 (Compressed): remove deletion-only hunks, drop tail files
  Stage 3 (Clipped): truncate large files at hunk boundaries
  Stage 4 (MultiCall): split into multiple batches (up to 5)

Integrated into review pipeline — files dropped by compression are
skipped with logging. 24 new tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: haasonsaas