feat: dedupe monorepo gems sharing identical changelog (#12)

[justi]

Summary

• Add _dedup_monorepo_deltas — pre-reflow awk pass that groups gems with byte-identical post-cutoff content under one comma-joined H3
• Reflow regex now accepts name1, name2: ... style headers
• Dedup is byte-level conservative: different bullets or version transitions stay split
• update_count computed before dedup, so the reported gem count is unaffected

Closes #12.

Test plan

• tests/test_save.sh extended: 9 new cases covering grouping, divergence (different bullets), and count integrity
• Full bash suite green: 12 test files, 169 cases, 0 regressions

Before / after

For a project with sentry-rails and sentry-ruby (both 5.23.0 → 6.5.0, identical bullets from monorepo):

-### sentry-rails
-
-**5.23.0 (last seen) → 6.5.0 (current, 2026-03-16)**
-
-- 6.5.0: Add OTLP collector_url; ...
-
-### sentry-ruby
-
-**5.23.0 (last seen) → 6.5.0 (current, 2026-03-16)**
-
-- 6.5.0: Add OTLP collector_url; ...
+### sentry-rails, sentry-ruby
+
+**5.23.0 (last seen) → 6.5.0 (current, 2026-03-16)**
+
+- 6.5.0: Add OTLP collector_url; ...

A typical Rails app saves ~3K tokens on the sentry-* family alone.

Notes

• Conservative criteria: only merges when (version transition string AND all bullets) match exactly. Won't false-positive on, e.g., rails meta-gem expansion since sub-gems differ on either dimension.
• No version bump in this PR.

🤖 Generated with Claude Code