feat: add Eval System for end-to-end agent evaluation

[singhhnitin]

Description

Implements the Eval System from the official roadmap — the first ever end-to-end agent graph evaluation framework for Aden Hive. Defines a new framework.eval module that lets developers benchmark agent quality across content correctness, latency, cost, tool usage, and semantic quality via LLM-as-judge.

Type of Change

• New feature (non-breaking change that adds functionality)

Related Issues

Closes eval-system item from roadmap.md

Changes Made

• New core/framework/eval/ module (6 files, zero new dependencies)
• EvalCase / EvalSuite — YAML-driven eval definitions with tag filtering and weighted scoring
• EvalScorer — multi-dimension scoring: content, latency, cost, tool usage, LLM-as-judge
• EvalRunner — async runner with concurrency control via asyncio.Semaphore
• EvalReport — aggregate report with JSON and Markdown export
• hive eval run and hive eval report CLI commands wired into framework/cli.py
• --fail-under flag for CI/CD pass rate gating
• Example eval suite at core/framework/eval/basic_agent_eval.yaml
• 17 unit tests covering all scoring dimensions

Testing

• Unit tests pass (cd core && pytest tests/test_eval/ — 17/17 passed)
• Lint passes (cd core && ruff check .)
• Manual testing performed

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Usage

hive eval run --suite core/framework/eval/basic_agent_eval.yaml --agent exports/my-agent --verbose
hive eval run --suite my_suite.yaml --agent exports/agent --fail-under 0.8 --output-json report.json
hive eval report report.json

[github-actions]

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Why is this required? See #472 for details.