fix(artifacts): write provider bundle snapshots

[christso]

Summary

• write generated task/eval provider snapshots as providers.yaml and expose them as providers_path in run indexes/manifests
• remove default targets_path/targets.yaml bundle read fallback for this hard migration
• migrate rerun/export/serve/validate fixtures, provider-file discovery, docs, and README to the current provider artifact contract

Validation

• bun run build
• bun test apps/cli/test/commands/eval/task-bundle.test.ts apps/cli/test/commands/eval/artifact-writer.test.ts apps/cli/test/commands/eval/bundle.test.ts apps/cli/test/commands/eval/targets.test.ts apps/cli/test/commands/runs/rerun.test.ts apps/cli/test/commands/results/export.test.ts apps/cli/test/commands/results/serve.test.ts apps/cli/test/commands/results/validate.test.ts (244 pass)
• bunx biome check <changed files>
• live dogfood via local OpenAI-compatible proxy: live provider live-openai plus live llm-rubric grader passed; verified .internal/index.jsonl contains providers_path, generated test/providers.yaml, and no targets_path/targets.yaml

Evidence

• Private evidence branch: agentv-private:evidence/av-kfik-51-provider-artifacts
• Evidence commit: d4d5157

Bead

• av-kfik.51

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