test: add Vitest setup with SiteManager and tool-registry coverage

[kleintech]

Summary

Establishes a foundation test suite using Vitest. ESM/TypeScript tests run via esbuild — no prior tsc build needed.

• SiteManager (17 tests) — env parsing (numbered WORDPRESS_N_* and legacy WORDPRESS_API_URL), getSite lookup with error paths, and detectSiteFromContext covering domain/alias/id matching, case-insensitivity, and null cases.
• Tool registry wiring (7 tests) — every registered tool has a corresponding handler, no orphan handlers, names are unique, every tool has a non-empty description and an object-shaped inputSchema with a properties record. Catches regressions when adding or renaming tools.
• GitHub Actions (.github/workflows/test.yml) — runs npm test on PRs and pushes to main on a Node 20 / 22 matrix.
• README — adds a short "Running Tests" subsection under Development.

npm test runs the full suite in ~500ms.

Notes

• The CI workflow uses npm install --ignore-scripts because the prepare hook runs npm run build, which currently fails on a fresh install due to axios type drift in media.ts and a TS2589 in server.ts. Those are pre-existing and unrelated to this PR — happy to fix them in a separate one.
• Initial scope is intentionally narrow ("foundation only"): pure-ish unit coverage with no HTTP mocking. Handler-level tests against a mocked WordPress API are a natural follow-up.

Test plan

• npm test passes locally (24/24)
• npm run test:watch re-runs on file changes
• CI green on Node 20 and 22

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