Migrate test runner from Jest to vitest

Why

The wizard's source is already ESM ("type": "module" in package.json), but the test runner is still Jest. Jest's ESM support is opt-in and finicky, which is creating real friction now that the wizard depends on ESM-native packages.

Concrete pain point

The __mocks__/@posthog/warlock.ts file exists solely because Jest can't cleanly load the real @posthog/warlock package — it's ESM-only and loads a YARA-X WASM binary at runtime. The mock substitutes hand-written jest.fn() stand-ins for the real engine just so tests can import without crashing.

Sibling-repo precedent

Both PostHog projects in the same neighborhood already use vitest:

• warlock: "test": "vitest run"
• context-mill: "test": "vitest run scripts/plugins/tests scripts/lib/tests"

A migration would unify test tooling across the three repos.

What's in scope

• Swap jest for vitest in devDependencies and package.json config
• Convert jest.fn() / jest.mock() calls to vi.fn() / vi.mock() (mostly mechanical)
• Re-evaluate the warlock mock — vitest's native ESM may eliminate most of it. (WASM-in-test is its own conversation; partial mock may still be needed.)
• Make sure the smoke-test step in the postbuild flow still runs

What's out of scope

• Browser-environment changes
• Test rewrites beyond the framework swap

Start with package.json to review the current Jest dependency, configuration, and test and postbuild scripts. Inspect mocks/@posthog/warlock.ts and the existing Jest calls, then run the test command and postbuild smoke-test step. Done means the Vitest suite passes, the smoke test still runs, and the warlock mock is reduced only where the migration permits.