[DRAFT] fix(#21): deprecate paper7 repo (Papers With Code API discontinued)

[Vigtu]

⚠️ Draft for discussion. This PR removes a feature. I want to align on whether deprecation is the right call before merging.

TL;DR

paper7 repo has been broken since Papers With Code's API went away (the brainstorm session originally planned to replace it with the Hugging Face Papers API). After implementing and probing, I confirmed there is no drop-in replacement for the curated paper-to-code mapping PWC published. This PR deprecates the command with a clear notice instead of substituting a worse data source.

The discussion question

Three options were on the table:

1. Deprecate cleanly ← this PR
2. Best-effort regex scan for github.com/* in HF Papers summary text (~10–30% coverage)
3. GitHub Search API (auth required, noisy results)

I went with (1) because (2) is a worse product (silent low coverage looks like bugs) and (3) is a heavier change with mediocre signal. If you'd rather keep the command working with reduced coverage, say so and I'll re-scope to option (2) — happy to add a paper7 repo --best-effort mode.

Evidence the API is gone (all probed live during implementation)

Probe	Result
paperswithcode.com/api/v1/*	HTTP 302 → huggingface.co/papers/trending
paperswithcode.com/api/v0/, v2/, /graphql, /search, /api/v1/datasets/	All redirect to trending
paperswithcode.com/paper/<slug> (HTML page)	All redirect to trending
HF GET /api/papers/<id> (BERT 1810.04805)	Fields: id, authors, publishedAt, summary, title. No GitHub field
HF GET /api/papers/<id> (ReAct 2210.03629)	Plus ai_summary, ai_keywords, upvotes. No GitHub field
HF GET /api/papers/<id>/repos, /models, /spaces, /datasets	404
HF /api/models?paper=<id>	Ignores filter (returns trending global)
Scrape huggingface.co/papers/2210.03629 for github.com/* URLs	0 hits
Semantic Scholar externalIds (Transformer 1706.03762)	MAG, DBLP, ArXiv, CorpusId — no GitHub
OpenAlex (ReAct via DOI)	Extensive metadata, no code-links field

What this PR changes

• Deletes src/repo.ts (the PWC client + decoder, 318 lines).
• src/commands/repo.ts becomes a one-shot deprecation message — no clients, no Effect environment requirements.
• paper7 repo runs with or without an arxiv/pubmed/doi id (the id arg is now optional and ignored). Existing script invocations like paper7 repo 2210.03629 still work, they just print the deprecation notice.
• Removes RepositoryDiscoveryClient / RepositoryDiscoveryLive / RepositoryDiscoveryError from cli.ts, including the formatRepositoryDiscoveryError handler and the Effect.provide(RepositoryDiscoveryLive) from main.
• Strips the unused PWC service mocks from browse.test.ts, cache.test.ts, vault.test.ts (they only ever provided an "unused" stub).
• Rewrites tests/refs-repo.test.ts: drops the PWC integration tests and the PWC error-class assertions; adds two deprecation tests.

User-facing output

$ paper7 repo 2210.03629
paper7 repo is deprecated.

The Papers With Code API has been discontinued (the domain now redirects to
huggingface.co/papers). Hugging Face acquired the brand but did not republish
the curated paper-to-code mapping, so paper7 has no drop-in source for
automatic repository discovery.

Most papers link their reference implementation in the abstract or first page.
Read the paper directly with:
  paper7 get <id> --abstract-only

Test plan

• npx vitest run → 165 pass / 0 fail (-3 from baseline: removed 5 PWC integration tests, added 2 deprecation tests).
• npx tsc --noEmit clean.
• New tests cover paper7 repo with and without an id, both producing the deprecation notice + exit 0.

CHANGELOG note for next release

• paper7 repo is now deprecated. The Papers With Code API was discontinued. No drop-in replacement is currently available. Try paper7 get <id> --abstract-only for paper metadata, or check the abstract for a code link.

Closes #21.