Fix metrics JSONL string bounds

[Widthdom]

Summary

• Clamp metrics JSONL string fields before serialization and emit <field>_length / <field>_truncated metadata.
• Keep serialized metrics events within an 8 KiB byte budget, including escaped-content cases.
• Document the truncation contract in USER_GUIDE.md and add the changelog fragment.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~MetricsSinkTests" -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet build CodeIndex.sln -p:UseSharedCompilation=false
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• codex exec review --base origin/main found missing metrics truncation docs; fixed in follow-up commit.
• codex exec review --ignore-user-config --ephemeral --base origin/main found no blocking/actionable regression.

Documentation / Changelog

• Updated USER_GUIDE.md.
• Added changelog.d/unreleased/3224.fixed.md.

Fixes #3224

Follow-up candidates

None.