Keep static commands usable with malformed project config

[Widthdom]

Summary

• classify CLI commands by project-config dependency so static metadata and help commands remain usable when project config is malformed
• return the stable E024_CONFIG_INVALID JSON envelope for config-dependent commands that select a JSON output contract
• preserve valid project-config environment settings for self-managed config commands while letting those commands report malformed config through their own contracts
• cover global option values, nested config flags, explicit command identity, workspace config paths, and help isolation with regression tests

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -f net8.0 --filter 'FullyQualifiedName~CdidxConfig' --no-restore --nologo (58 passed)
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -f net9.0 --filter 'FullyQualifiedName~CdidxConfig' --no-restore --nologo (58 passed)
• dotnet test CodeIndex.sln -f net8.0 --no-restore --nologo (10,880 passed, 7 skipped)
• dotnet test CodeIndex.sln -f net9.0 --no-restore --nologo (10,406 passed, 420 skipped; clean rerun after one unrelated MCP metrics test transiently failed and passed in isolation)
• dotnet build CodeIndex.sln -c Release --no-restore --nologo
• dotnet build CodeIndex.sln -c Debug --no-restore --nologo
• dotnet format CodeIndex.sln --no-restore --verify-no-changes --verbosity minimal
• dotnet run --project tools/CodeIndex.Changelog -- check
• Codex adversarial review: two rounds completed; all actionable findings addressed

Documentation and changelog

• updated the English and Japanese sections of USER_GUIDE.md and DEVELOPER_GUIDE.md
• added bilingual fragment changelog.d/unreleased/4886.fixed.md

Follow-ups

• None.

Fixes #4886