Stabilize non-database CLI error envelopes

[Widthdom]

Summary

• Add a shared stable JSON error envelope for non-database CLI failures.
• Route outline, hooks, doctor, and validate-config failures through semantic error codes, stable exit metadata, safe diagnostics, and consistent stdout/stderr behavior.
• Add table-driven regression coverage for the supported command and failure combinations.

Root cause and impact

These commands previously constructed errors independently, so JSON fields, codes, hints, command context, exit metadata, and stream behavior varied by command and failure path. Scripts can now consume a consistent contract while human-readable invocations retain concise diagnostics.

Validation

• dotnet build CodeIndex.sln -c Release --no-restore
• dotnet build CodeIndex.sln -c Debug --no-restore
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check
• Focused Release contract and hooks tests on net8.0 and net9.0
• Manual JSON-path sanitization and stdout/stderr checks
• Two adversarial Codex review rounds; all actionable findings addressed
• Full solution test run attempted; unrelated follow-up is tracked in Stabilize the batch timeout concurrency test under parallel suite execution #4947

Documentation and changelog

• Updated the English and Japanese sections in README.md, USER_GUIDE.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md.
• Added changelog.d/unreleased/4855.fixed.md.

Follow-up

• Stabilize the batch timeout concurrency test under parallel suite execution #4947

Fixes #4855