feat(cli): --threshold flag for suite-level quality gates

[christso]

Objective

Add a --threshold CLI flag to agentv eval that fails (exit 1) if the mean score across all tests falls below the specified threshold. This enables CI/CD quality gating without needing agentv compare --baseline.

Design Latitude

• Add --threshold <number> flag (0-1 scale) to agentv eval
• Also support execution.threshold in EVAL.yaml for per-suite defaults
• CLI flag overrides YAML value
• After all tests complete, compute mean score; if below threshold, exit code 1 with summary
• Integrate with existing JUnit XML output (test-level pass/fail based on threshold)

Acceptance Signals

• agentv eval evals/ --threshold 0.6 exits 1 if mean score < 0.6
• execution.threshold: 0.6 in YAML has the same effect
• CLI --threshold overrides YAML execution.threshold
• Summary line printed: "Suite score: 0.53 (threshold: 0.6) — FAIL"
• Exit code 0 when score meets threshold

Non-Goals

• Not a replacement for agentv compare regression gating (different use case)
• Not per-test threshold override (use required for that)
• Not severity levels (feat(eval): composable quality gates with auto-remediation triggers #334 covers that separately)

Context

Identified via microsoft/skills harness research. The skills harness uses --threshold 60 (0-100 scale) for CI quality gates. AgentV currently has per-test required gates and agentv compare --baseline regression gating, but no suite-level threshold.