feat: gem-contribute rate <gem|owner/repo> — Good First Repo scoring

[cdhagmann]

Summary

Add a rate command that scores a single gem (or owner/repo) against a "Good First Repo" rubric and prints a breakdown. Proof-of-concept for the scoring logic before review (batch mode across a full lockfile) is built.

gem-contribute rate zeitwerk
gem-contribute rate rails/rails

Motivation

Based on Chael's talk ("find a Good First Repo, not just a Good First Issue"). Most of the raw signal is already fetchable via HostAdapter#community_profile and the existing issues fetch — this is mostly a display and aggregation problem.

Signals

Welcoming (from community_profile — one cached call):

• README, CONTRIBUTING, CoC, license, issue template, PR template — present/absent
• GitHub's health_percentage as a sanity baseline

Active (one additional call — latest commit on default branch):

• Days since last commit on default branch
• Open PR count + recent throughput

Issue quality (from the existing issues fetch):

• Count of open good first issue / help wanted issues
• Age of the oldest still-open good-first-issue (stale backlog = red flag)

Display

Two labeled axes rather than a single grade — respects the user's judgment and matches the project's "render, don't editorialize" ethos (ADRs 0005 + 0007):

zeitwerk — Good First Repo rating
  Welcoming  ████████░░  80%   README ✓  CONTRIBUTING ✓  CoC ✗  issue template ✓  PR template ✗
  Active     ██████████  last commit 2d · 3 open PRs · 2 merged last 30d

  3 open "good first issue" issues · oldest 14d
  To contribute: gem-contribute fix zeitwerk/<issue#>

Notes

• Composable with future review command: review will just run rate over every github.com gem in the lockfile.
• Needs an ADR before implementation — signal selection and the two-axis display shape both brush against ADR-0005 and ADR-0007.
• Self-rating use case: gem-contribute rate gem-contribute as a forcing function to ensure this repo would score well on its own rubric.
• Use allow_owner_repo: true in resolve_target — already supported by the Workflow mixin.

Out of scope for this issue

• Batch mode across a lockfile (review command — separate issue)
• Persisting scores or comparing across time
• GitLab / Codeberg (post-v1 host adapters)