fix(core): clarify use_target resolution errors

[christso]

Summary

• resolve use_target aliases through a shared helper instead of letting unresolved aliases fall through to the generic provider error
• surface actionable errors when an env-backed alias is unset or resolves to a missing target
• add regression coverage for delegated target resolution

Red/Green UAT

Red:

• From D:/GitHub/WiseTechGlobal/WTG.AI.Prompts
• Command: $env:GRADER_TARGET=''; agentv eval evals/cargowise/cw-refvessel.eval.yaml --workers 1
• Result: repeated execution errors like grader: 'provider' is required (targets with use_target must be resolved before calling resolveTargetDefinition)

Green:

• From D:/GitHub/WiseTechGlobal/WTG.AI.Prompts
• Command: $env:AGENT_TARGET='copilot'; $env:LLM_TARGET='azure'; $env:GRADER_TARGET=''; bun D:/GitHub/christso/agentv.worktrees/fix-helpful-use-target-error/apps/cli/dist/cli.js eval evals/cargowise/cw-refvessel.eval.yaml --target copilot --workers 1
• Result: repeated execution errors like Target "grader" uses use_target: ${{ GRADER_TARGET }}, but GRADER_TARGET is not set. Set GRADER_TARGET to the name of a concrete target (for example, "azure") before running the eval.

Verification

• bun test packages/core/test/evaluation/providers/targets.test.ts
• bun test packages/core/test/evaluation/orchestrator.test.ts
• bun --filter @agentv/core typecheck
• pre-push hook on git push

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