Fix shell completion option parity

[Widthdom]

Summary

• register the parser-accepted lsp, workspace, config, diff, import, and export options in the authoritative CLI flag schema, including exact nested/default contexts and the command-scoped -o alias
• generate context-exact candidates for Bash, zsh, fish, and PowerShell while preserving the existing hooks status PowerShell contract
• add bidirectional parser/help/schema/renderer coverage, including positive and negative nested cases, value metadata, aliases, diff mode conflicts, and generated-script execution

Validation

• dotnet restore CodeIndex.sln
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore (0 warnings, 0 errors)
• affected completion/help tests on net8.0 and net9.0 (311 passed on each final run)
• full regression runs before the review follow-ups: net8.0 (12,023 passed, 9 skipped) and net9.0 (11,539 passed, 430 skipped)
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check
• root and manifest workspace index health checks
• two Codex adversarial-review rounds; all findings were fixed and the affected tests were rerun

Documentation and changelog

• updated the English and Japanese shell-completion contracts in DEVELOPER_GUIDE.md
• updated the English and Japanese test guidance in TESTING_GUIDE.md
• added changelog.d/unreleased/5194.fixed.md

Follow-up candidates

None.

Fixes #5194