refactor(core): rename Evaluator to Grader across codebase

[christso]

Closes #1109

Summary

This is the hard rename from Evaluator to Grader across the codebase.

• Renames core interfaces, classes, registries, loaders, exports, tests, and directory layout from Evaluator* / evaluators/ to Grader* / graders/
• Removes the old defineCodeJudge / CodeJudgeHandler / runCodeJudge-style aliases and related evaluator-era exports
• Updates CLI, Studio, docs, examples, plugins, and AGENTS guidance to use grader terminology
• Preserves YAML kind strings, scores[].type, evaluation/, and the evaluate() API

Breaking change

This PR intentionally hard-removes the old TypeScript symbol names and deep import paths. There are no backcompat aliases.

Old symbol	New symbol
Evaluator (interface)	Grader
EvaluatorKind	GraderKind
EVALUATOR_KIND_VALUES	GRADER_KIND_VALUES
EvaluatorRegistry	GraderRegistry
FieldAccuracyEvaluator	FieldAccuracyGrader
SkillTriggerEvaluator	SkillTriggerGrader
CodeEvaluator	CodeGrader
LlmGraderEvaluator	LlmGrader
ToolTrajectoryEvaluator	ToolTrajectoryGrader
TokenUsageEvaluator	TokenUsageGrader
LatencyEvaluator	LatencyGrader
CostEvaluator	CostGrader
InlineAssertEvaluator	InlineAssertGrader
CompositeEvaluator	CompositeGrader
ExecutionMetricsEvaluator	ExecutionMetricsGrader
DeterministicAssertionEvaluator	DeterministicAssertionGrader
defineCodeJudge (@agentv/eval)	defineCodeGrader
Import path @agentv/core/.../evaluators/...	.../graders/...

Verification

• bun run build
• bun run typecheck
• bun run lint
• bun run test
• bun run validate:examples
• Live eval before/after comparison using examples/features/execution-metrics/evals/dataset.eval.yaml --test-id simple-thresholds-pass confirmed identical scores[].type output (["execution-metrics"]) on origin/main and this branch
• Studio smoke-check confirmed the eval detail view now renders Grader Scores with no stale evaluator label in visible UI copy

Live eval artifacts

• Before: /home/christso/.copilot/session-state/264152e9-2410-40ef-9068-b7b31da6998e/files/issue-1109-live-eval/main/index.jsonl
• After: /home/christso/.copilot/session-state/264152e9-2410-40ef-9068-b7b31da6998e/files/issue-1109-live-eval/worktree/index.jsonl

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