[codex] Fix inspect flags and help coverage

[Widthdom]

Summary

• Add inspect source excerpt support for --line, range, context, and path/line-only usage.
• Let inspect --kind filter definition candidates and prefer production source definitions over matching test definitions.
• Add --body-line-count as an alias for inspect --body-lines.
• List every implemented command in the top-level cdidx --help command summary.

Issues

Fixes #3786
Fixes #3666
Fixes #3665
Fixes #3663

Validation

• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check origin/main..HEAD
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~Issue
• dotnet build CodeIndex.sln
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll inspect RunInspect --line 5363 --context 1 --json --fields source_excerpt --exact-name
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll inspect --path src/CodeIndex/Cli/QueryCommandRunner.cs --line 5363 --json --fields source_excerpt

Review

Adversarial review of origin/main..HEAD: no blocking/actionable issues found.

Notes

• Existing issue comments were checked before starting; none of Show source excerpts and line ranges from cdidx inspect #3786, Let inspect prioritize source definitions and filter by kind or source/test scope #3666, Add inspect option aliases or suggestions for body-line flags #3665, or List all implemented commands in top-level help #3663 had comments indicating someone else had started work.
• No additional issues were opened.