docs(examples): clarify scenarios matrix use case

[christso]

Summary

• revise the scenarios feature example to demonstrate one shared test set crossed with multiple language config rows
• update scenario docs/README so the use case is visually clear and not just a parser smoke test
• add durable AGENTS/.agents verification guidance that feature dogfood must prove the real use case
• fix the CLI terminal summary count exposed by the scenario matrix dogfood run

Validation

• bun apps/cli/src/cli.ts validate examples/features/scenarios/evals/suite.yaml
• bun apps/cli/src/cli.ts eval run examples/features/scenarios/evals/suite.yaml --targets examples/features/scenarios/targets.yaml --target translation-cli --output /tmp/agentv-scenarios-matrix-fixed-BpmNJV
  • printed RESULT: PASS (6/6 scored >= 80%) and Total tests: 6
• bun run validate:examples
• bun test apps/cli/test/commands/eval/aggregate.test.ts
• bun run lint
• bun run build
• git diff --check

Closes av-2inz
Closes av-bins

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