Document reviewer tool evidence lifecycle metadata

[zzwong]

Problem

reviewer_tool_evidence is durable lifecycle metadata that affects resumed coverage and approval behavior, but the canonical task-artifact schema documentation does not describe the field.

Impact

Maintainers cannot determine its status values, whether it is load-bearing, or how version-1 artifacts without the field remain compatible.

Acceptance criteria

• Document reviewer_tool_evidence in docs/llm-task-artifacts.md.
• Describe its status values and how they affect resume, coverage, and approval behavior.
• State whether the field is load-bearing.
• Explain version-1 compatibility for artifacts that do not contain the field.

Evidence: PR #540 review at internal/llmlifecycle/lifecycle.go.