feat(sdk): add CheckContext.repoRoot for zoned runs (TV2.3)

[yan-vidal]

TV2.3 — SDK `CheckContext.repoRoot`

Additive SDK field that unblocks Phase V2 per-zone execution (TV2.2) and the Phase V3 cross-language proof (`check-clippy`, TV3.2). First step of the v2 multi-language work toward running Sentiness on non-JS projects (e.g. Rust).

Change

• `CheckContext` now carries `readonly repoRoot: string` alongside `cwd`:
  • `cwd` is the zone root (per-zone in a polyglot monorepo).
  • `repoRoot` is the repository root for the rare check that needs repo-level context.
• Single-zone today, so the runner and `doctor` set `repoRoot = cwd`. TV2.2 will set `cwd = zone.absRoot` while keeping `repoRoot` at the repo root.
• All check `makeContext` test helpers mirror it (`repoRoot = cwd`), so existing suites pass unchanged.

Scope

Strictly additive (OCP). No behavior change for single-zone projects — it only threads a new field that future zoned execution will populate differently.

Validation

• `pnpm -r typecheck` ✅
• `pnpm -r test` ✅ (core 209, SDK 13 incl. the new `repoRoot` type test, all checks + cli)
• `pnpm lint` ✅ (219 files)
• `pnpm build` ✅
• Dogfood `pnpm sentiness check --tier=fast` → `status: ok`, non-blocking

Deviations from spec

The TV2.3 spec mentions updating a shared `_test-utils` `makeContext`; in practice each check test owns a local `makeContext` helper (no shared one exists), so each was updated individually. End state is equivalent.