Add the deterministic, idempotent seed that brings a freshly provisioned
throwaway repository to a known ground truth before a trial runs, scripted
over the live Gitea API.
- bench/seed-plan.ts: the pure ground truth (fixed labels, an issue spread
across the discriminating dimensions, and labelled/reviewed/real-branch
pull requests) plus groundTruth(user), realizing it into a RepoState.
- bench/seed.ts: idempotent seeding reconciled by natural key, reusing
gitea-axi's own tea-login credential discovery (no new secret handling).
- A live smoke tier (test:bench:smoke) validating the seed end-to-end and
skipping cleanly when no host is configured, kept out of the deterministic
bench tier.
Export selectLogin from src/context.ts so the bench reuses the exact
credential-selection path.
