Surface unsupported files in index diagnostics

[Widthdom]

Summary

• Surface bounded unsupported-language coverage diagnostics in full index completion, dry runs, watch initialization, persisted status, and compact status output.
• Group unmapped files by normalized extension and category with capped samples, omission metadata, actionable guidance, and lower-bound signaling for incomplete scans.
• Include actionable extensionless files while keeping ignored, binary, oversized, unreadable, and internal-artifact files out of language-support warnings.
• Version persisted diagnostics so legacy counts are omitted until a current full scan refreshes them, and preserve conservative trust behavior for partial archives.

Validation

• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false (0 warnings, 0 errors)
• dotnet test CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false
  • net8.0: 11,738 passed, 7 skipped, 0 failed
  • net9.0: 11,259 passed, 423 skipped, 0 failed
• Issue Surface unsupported files in index completion diagnostics #5100 targeted tests on net8.0 and net9.0: 13 passed per target, 0 failed
• Archive/manifest compatibility target set: 16 total per target; net8.0 16 passed, net9.0 15 passed / 1 environment-conditional skip
• dotnet format CodeIndex.sln --no-restore --verify-no-changes
• dotnet run --project tools/CodeIndex.Changelog -- check
• Root cdidx status --check and workspace manifest check both healthy

Review

• Completed two Codex adversarial-review rounds and addressed all actionable findings.

Documentation and changelog

• Updated the English and Japanese documentation sections in README.md, USER_GUIDE.md, and DEVELOPER_GUIDE.md.
• Updated the repository status contract in AGENT_GUIDE.md.
• Added bilingual changelog fragment changelog.d/unreleased/5100.changed.md.

Fixes #5100

Follow-up: #5139