fix(core): allow templated use_target validation

[christso]

Bead: av-njl

Summary:

• Accept env-templated use_target values during targets.yaml validation even when the referenced env vars are unset.
• Preserve provider validation for targets that do not supply a concrete or templated use_target.
• Add regression coverage for unresolved AGENT_TARGET and GRADER_TARGET.

Verification from worker handoff:

• bun test packages/core/test/evaluation/validation/targets-validator.test.ts
• bun --filter @agentv/core typecheck
• bun --filter @agentv/core test
• bun run test
• bun run validate:examples

Integration review: diff checked against current origin/main; no PR existed before this one.