feat(cli): infer Vitest verifier eval commands

[christso]

Summary

• infer the Vitest code-grader adapter for verifier-looking files passed to agentv eval
• keep agentv eval vitest as the explicit adapter command for advanced flags
• update docs, eval-writer guidance, and the Vitest workspace grader example to use the shorthand

Detection

• *.test.ts/js/tsx/jsx and *.spec.ts/js/tsx/jsx
• Vercel-style EVAL.ts/js/tsx/jsx
• *.eval.ts and YAML eval files still route to normal eval run

Verification

• bun test apps/cli/test/unit/preprocess-argv.test.ts
• bun test apps/cli/test/commands/eval/vitest.test.ts
• bun apps/cli/src/cli.ts eval examples/features/vitest-workspace-grader/evals/dataset.eval.yaml --target mock_agent
• bun --filter @agentv/core build (to refresh dist after pulling current main)
• bun --filter agentv test
• bun --filter agentv typecheck
• bun --filter agentv lint
• bun --filter agentv build
• bun run validate:examples

Tracker: av-bv4.20

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