feat: add benchmark seed provisioning (task 0025)

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.

package.json

@@ -35,7 +35,8 @@
     "test:coverage": "vitest run --coverage",
     "test:e2e": "vitest run --config vitest.e2e.config.ts",
     "test:pack": "vitest run --config vitest.packaging.config.ts",
-    "test:bench": "vitest run --config vitest.bench.config.ts"
+    "test:bench": "vitest run --config vitest.bench.config.ts",
+    "test:bench:smoke": "vitest run --config vitest.bench-smoke.config.ts"
   },
   "dependencies": {
     "@toon-format/toon": "^2.3.0",