fix(results): disambiguate target artifact directories

[christso]

Summary

Multi-target runs that reuse the same suite and test ID now write separate physical artifact directories instead of overwriting the same run-1 folder. Single-target paths keep the existing suite/test/run-N or test/run-N layout, and index.jsonl remains the authoritative source for every emitted path.

Validation

• Red check before the fix: bun test apps/cli/test/commands/eval/artifact-writer.test.ts --test-name-pattern "allocates distinct artifact directories" failed with both rows pointing at suite-a/shared-id.
• Green focused checks: bun test apps/cli/test/commands/eval/artifact-writer.test.ts, bun test apps/cli/test/commands/eval/aggregate.test.ts, bun test packages/core/test/evaluation/orchestrator.test.ts.
• Static checks: bun --filter @agentv/core typecheck, bun --filter agentv typecheck, bunx biome check packages/core/src/evaluation/run-artifacts.ts apps/cli/test/commands/eval/artifact-writer.test.ts, git diff --check.
• Live dogfood: bun apps/cli/src/cli.ts eval run /tmp/agentv-av9919-live-Uxi4Gm/target-artifact-allocation.eval.yaml --targets /tmp/agentv-av9919-live-Uxi4Gm/targets.yaml --target baseline --target candidate --workers 1 --output /tmp/agentv-av9919-live-Uxi4Gm/results passed 2/2 with real OpenAI-backed targets and LLM grading. The manifest recorded shared-id/baseline and shared-id/candidate artifact dirs.

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[christso]

Closing as superseded by PR #1540 and the av-504 result_dir contract direction. PR #1538 used the older target artifact allocation approach and has failing tests; any useful target-disambiguation ideas should be carried forward through #1540/#1541 instead.