[codex] Split diff and export/import helpers

[Widthdom]

Summary

• Split diff option parsing and result rendering out of DiffCommandRunner.
• Split export/import manifest decoding/header validation and nullable SQLite row reads out of ExportImportCommandRunner.
• Added focused helper tests for parser errors, diff summary JSON, manifest malformed/over-depth metadata validation, and nullable row reads.

Validation

• dotnet build tests/CodeIndex.Tests/CodeIndex.Tests.csproj --framework net8.0 --no-restore -p:RunAnalyzers=false -p:UseSharedCompilation=false -p:BuildInParallel=false -m:1 -nodeReuse:false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --framework net8.0 --no-build --no-restore --filter "FullyQualifiedName~DiffCommandHelpersTests|FullyQualifiedName~ExportImportManifestCodecTests" -p:UseSharedCompilation=false -nodeReuse:false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --framework net8.0 --no-build --no-restore -p:UseSharedCompilation=false -nodeReuse:false
• dotnet run --project tools/CodeIndex.Changelog/CodeIndex.Changelog.csproj --framework net8.0 --no-build -- check
• git diff --check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index /Users/widthdom/Projects/mine/cdidx/CodeIndex18th
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review: no actionable regressions found in the staged/untracked changes.

Documentation / Changelog

• Added changelog.d/unreleased/4181.internal.md.
• No user-facing CLI contract changed, so no README/User Guide update was needed.
• AGENTS.md, CLAUDE.md, and AGENT_GUIDE.md were not modified.

Follow-up Candidates

• Broader export/import orchestration splits can continue in later issues if needed.

Fixes #4181