Background

gem-contribute scan currently filters open issues by the single canonical label "good first issue" (see Scan::DEFAULT_LABEL). Many repos use slight variants of this label and are silently dropped from the ranked list.

Concrete example: rainbow's issue #112 is labeled "Good first issue :tada:" (note the suffix and capitalization). It does not match "good first issue" exactly and never appears in scan output, even though it's a perfect contribution target.

ADR-0005 deliberately rejects label normalization (don't rewrite "easy" → "beginner-friendly") — but it explicitly carves out preferred_labels as the user-controlled escape valve:

preferred_labels matches case-insensitively against the raw label text. Hyphens and spaces are treated as equivalent.

We have not implemented preferred_labels yet. This issue is that work.

Desired behavior

1. Add preferred_labels to Config with a sensible default. Suggested default:

   preferred_labels:
     - good first issue
     - good-first-issue
     - help wanted

2. gem-contribute config set preferred_labels "label1,label2" (comma-separated; YAML list under the hood).
3. Scan runs one API request per preferred label, dedupes by issue number, and ranks the union.
4. Matching is case-insensitive; - and are interchangeable. The label "Good first issue :tada:" matches "good first issue" if (and only if) we widen the comparison to a substring or normalize the suffix away — discuss in the PR which semantics you choose.

Acceptance criteria

• preferred_labels is read from ~/.config/gem-contribute/config.yml.
• Scan queries each label and merges results, deduping by (owner, repo, number).
• Cache key in GitHubAdapter#issues continues to work for the multi-label case (already keyed on sorted label list).
• Scanning a lockfile that includes rainbow surfaces issue #112 (or whatever the current set is) by default, without further configuration.
• Specs cover: empty config falls back to defaults, custom config overrides defaults, dedupe is by number not label.
• No regression in gem-contribute issues <gem> (which uses the same label list).

Out of scope

• Synonym tables ("easy" ≈ "beginner") — that's the normalization ADR-0005 rejects.
• A full label-discovery UI in the TUI — Stage 3 territory.

Files likely to touch

• lib/gem_contribute/config.rb — add the key, default
• lib/gem_contribute/cli/scan.rb — replace DEFAULT_LABEL with the configured list
• lib/gem_contribute/cli/issues.rb — same
• spec/gem_contribute/cli/scan_spec.rb, spec/gem_contribute/cli/issues_spec.rb, spec/gem_contribute/config_spec.rb

How to pick this up

gem-contribute fix gem-contribute/<this-issue-number>

This is a workshop-friendly first contribution.