Document and gate relaxed JSONL encoder use

[Widthdom]

Problem

UnsafeRelaxedJsonEscaping is used for local JSONL sinks. The current comments say these streams are local-only, but the safety boundary is implicit and could be weakened by future reuse.

Evidence

• Mcp/AuditLogSink.cs configures JavaScriptEncoder.UnsafeRelaxedJsonEscaping.
• Cli/MetricsSink.cs configures JavaScriptEncoder.UnsafeRelaxedJsonEscaping.
• Both appear intended for local append-only JSONL, not HTML or remote embedding.

Suggested fix

• Centralize or clearly name the local-only JSONL serializer options.
• Add comments/tests that assert these options are not used for HTTP responses, HTML contexts, or externally embedded JSON.
• Consider a small guard or type distinction for local telemetry/audit JSONL output.

[Widthdom]

Work started on branch fix-issue3965.