Document missing command error codes

[Widthdom]

Summary

• document E015_FS_CASE_PROBE_FAILED, E016_CHECKPOINT_NOT_FOUND, and E017_WORKSPACE_MANIFEST_INVALID in both USER_GUIDE error-code tables
• add a documentation contract test that keeps both language tables synchronized with CommandErrorCodes
• add the bilingual changelog fragment at changelog.d/unreleased/4644.docs.md

Why

The stable command error-code taxonomy already defined E015–E017, but the English and Japanese USER_GUIDE tables skipped directly from E014 to E018. This left scripts, AI agents, and users without a complete reference.

Impact

The user guide now exposes the complete stable error-code sequence in English and Japanese. Future additions or removals from CommandErrorCodes will fail the documentation drift test unless both tables are updated in the same change.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-restore --filter FullyQualifiedName~CodeIndex.Tests.DocumentationDriftTests
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review of origin/main..HEAD: no blocking/actionable issues found

An additional full-solution test run was stopped after 22 minutes while sandbox-conditioned tests continued to be skipped; no failures were reported before cancellation. The focused documentation drift suite passed on both net8.0 and net9.0.

Documentation and changelog

• updated USER_GUIDE.md in both English and Japanese sections
• added changelog.d/unreleased/4644.docs.md

Follow-up candidates

None.

Fixes #4644