Fix batch json-summary envelopes (#4142)

[Widthdom]

Summary

• Changed cdidx batch --json-summary to emit per-line machine-readable batch_result / batch_error records and a final batch_summary record.
• Captured child command stdout/stderr inside each record instead of mixing it into top-level process output.
• Added coverage for malformed JSON lines, unsupported batch commands, child command failures, and captured output limit handling.

Documentation and changelog

• Updated USER_GUIDE.md, DEVELOPER_GUIDE.md, and CLI help text for the new --json-summary contract.
• Added changelog fragment changelog.d/unreleased/4142.fixed.md.
• AGENT.md, CLAUDE.md, and AGENT_GUIDE.md were not changed.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false -p:BuildInParallel=false -m:1 --no-restore
• dotnet tools/CodeIndex.Changelog/bin/Release/net8.0/CodeIndex.Changelog.dll check
• git diff --check
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --no-build --filter "FullyQualifiedName~QueryCommandRunnerTests.RunBatch_" (net8.0 and net9.0)
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

Review

• Codex adversarial review: no blocking/actionable issues identified.

Fixes #4142