[Agents] Add Voice diagnostics and turn metrics changelog

[AbdulsaboorS]

Summary

• announce per-turn metrics and browser diagnostics in @cloudflare/voice v0.4.0
• explain how developers can inspect turn outcomes and timing stages
• document the opt-in browser diagnostics configuration and privacy boundary

Related

• Improve voice diagnostics and turn lifecycle visibility agents#2157
• @cloudflare/voice v0.4.0

Review

This draft is ready for technical review from @cjol. Please confirm the metrics semantics, diagnostics and redaction claims, and package imports before publication.

Validation

• pnpm run check
• pnpm run lint
• pnpm run format:core:check
• pnpm run build
• git diff --check

[cloudflare-docs-bot]

Review

✅ No issues found in commit 56f8393.

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[korinne]

Ship!