Make workspace status CI-actionable

[Widthdom]

Summary

• distinguish project-directory and database existence with sibling project_exists / db_exists fields while retaining exists as a compatibility alias
• add workspace status --check with stable aggregate exits: healthy 0, missing 2, degraded 5, malformed input 1
• publish per-member reasons and structured repair actions plus aggregate counts, reasons, actions, and enforcement exit metadata
• cover missing/empty/malformed manifests, missing directories/databases, mixed health, shared databases, human/JSON output, and paths with spaces

Root cause

Workspace status exposed member exists for project-directory existence beside index_health.db_exists, and its informational success exit was the only behavior even when required member indexes were absent. Consumers therefore had neither an unambiguous existence contract nor a CI enforcement mode.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false (0 warnings, 0 errors)
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net8.0 --no-build --no-restore -p:UseSharedCompilation=false (10,868 passed, 7 skipped, 0 failed)
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net9.0 --no-build --no-restore -p:UseSharedCompilation=false (10,394 passed, 420 skipped, 0 failed)
• targeted WorkspaceCommandRunnerTests (70 passed)
• dotnet format CodeIndex.sln --no-restore --verify-no-changes
• dotnet run --project tools/CodeIndex.Changelog -- check
• Release CLI smoke for ordinary status, --check, JSON, and help
• Codex adversarial review: No blocking/actionable issues found.

Documentation and changelog

Updated the English and Japanese README, developer guide, testing guide, and changelog.d/unreleased/4885.changed.md.

Fixes #4885