fix(providers): emit skill call metadata

[christso]

Summary

• derive canonical skill call metadata from normalized provider messages and wire it through pi, copilot, cli, and SDK child providers
• separate confirmed skillCalls from attemptedSkillCalls for failed skill reads, following Promptfoo's useful metadata pattern
• hard-deprecate the legacy skill-trigger grader surface and reject authored skill-trigger with skill-used / not-skill-used migration guidance
• update LLM grader docs to reflect target-only grader config

Verification

• bun test packages/core/test/evaluation/providers/skill-calls.test.ts packages/core/test/evaluation/providers/pi-runtime.test.ts packages/core/test/evaluation/providers/cli.test.ts packages/core/test/evaluation/providers/copilot-sdk.test.ts packages/core/test/evaluation/providers/copilot-cli.test.ts packages/core/test/evaluation/providers/sdk-child-provider.test.ts
• bun test packages/core/test/evaluation/graders/promptfoo-assertions.test.ts packages/core/test/evaluation/loaders/eval-yaml-transpiler.test.ts packages/core/test/evaluation/loaders/grader-parser.test.ts packages/core/test/evaluation/validation/eval-file-schema.test.ts packages/core/test/evaluation/validation/eval-validator.test.ts
• bun run lint
• bun run typecheck
• live dogfood: Codex SDK target + local-openai-grader + skill-used assertion passed 1/1; bundle: .agentv/results/dogfood-av-xdwn/run-skill-used-codex-key

Notes

• Promptfoo reference checked locally at /home/entity/projects/promptfoo/promptfoo commit 6bfc5a0c7f16f9c4717ac731d276b578e63d0769.
• Existing local dogfood caveats observed: inline eval target api_key templates are rendered before target secret validation, so the dogfood used the existing local-openai-grader target with LOCAL_OPENAI_PROXY_* and CODEX_API_KEY env overrides.

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