[enhancement] Deduplicate monorepo gems sharing identical CHANGELOG

[justi]

Summary

When two gems are published from the same monorepo with a unified CHANGELOG (e.g. sentry-rails and sentry-ruby from getsentry/sentry-ruby), postcut emits identical bullet content for both gems. This wastes tokens in the generated LLM context document without adding information.

Observed

In a generated .postcut/<model>.md for a project depending on both sentry-rails and sentry-ruby, sections ### sentry-rails and ### sentry-ruby contain bit-identical bullet lists. Verified with:

diff <(awk 'BEGIN{p=0} /^### sentry-rails/{p=1;next} /^### /{if(p)exit} p' doc.md | grep "^- ") \
     <(awk 'BEGIN{p=0} /^### sentry-ruby/{p=1;next} /^### /{if(p)exit} p' doc.md | grep "^- ")
# → empty diff, 17 identical bullets each

Root cause

Confirmed: upstream getsentry/sentry-ruby monorepo publishes a unified CHANGELOG.md for the entire SDK ecosystem, with no per-gem section differentiation per version. Postcut faithfully reflects this — but for a project pulling both gems, the doc duplicates ~17 bullets × ~200 chars = ~3.4K tokens.

Comparison: rails meta-gem already handled

README.md notes that the rails meta-gem expands into per-component prefixes ([activerecord], [actionview], ...). This works well in current output. Sentry-style monorepos need similar treatment but inverted: instead of expanding one gem into many sub-sections, group multiple gems sharing a CHANGELOG under one section.

Suggested approaches

1. Group section header: ### sentry-rails, sentry-ruby with one bullet list.
2. Reference pointer: Emit full content for first gem alphabetically, then for siblings emit ### sentry-ruby\n\n*See [sentry-rails](#sentry-rails) — shared CHANGELOG from getsentry/sentry-ruby monorepo.*
3. Detect via shared upstream URL: when two gems' gem specification | homepage_uri points to the same repo AND their CHANGELOGs hash-match, treat as duplicates.

Impact

Token efficiency for in-flight LLM context. A typical Rails app may pull 3-5 monorepo siblings (e.g. sentry-*, pay-*, view_component-*) — savings compound.

🤖 Reported via Claude Code review