fix(artifacts): keep eval runs at timestamp root

[christso]

Summary

Finalize the v1 run artifact layout so a timestamped run bundle owns one root index.jsonl manifest and one root summary.json. New eval runs no longer fan out into target or variant directories; target/model/variant remain metadata, and per-result artifacts stay discoverable through explicit manifest fields like result_dir, summary_path, grading_path, metrics_path, and transcript_path.

What Changed

• Replaced the CLI target/variant bundle writer with a single run-root manifest writer in agentv eval.
• Updated resume/final artifact writing to aggregate from the run root and preserve allocated per-result directories.
• Added ADR 0012 documenting the finalized layout, opaque allocated result directories, legacy nested-manifest reader compatibility, and the future internal/ v2 option.
• Updated result artifact docs, concepts, and layout tests to lock the root-manifest contract.

Code Review

• Reviewed branch diff against origin/main with correctness, testing, maintainability, project-standards, agent-native, and learnings lenses.
• No actionable findings.
• Residual risk: writer integration coverage is updated but could not be executed in this checkout because missing dependencies prevent the suite from starting.

Validation

• bun test apps/cli/test/commands/eval/result-layout.test.ts passed.
• git diff --check passed.
• Attempted bun test apps/cli/test/commands/eval/artifact-writer.test.ts; it failed before running tests because @agentv/core could not be resolved.
• Attempted bun test apps/cli/test/eval.integration.test.ts; it failed before running tests because execa could not be resolved.
• Did not run bun install per repo instructions.

Post-Deploy Monitoring & Validation

No production service monitoring is needed. After merge, validate by running a fresh eval and confirming .agentv/results/<experiment>/<timestamp>/index.jsonl is the only new run manifest, with per-result sidecars resolved through row path fields and no target/model/variant/cases folder layer.

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