Expose MCP strict audit logging in help and completions

[Widthdom]

Summary

• register every public mcp option in the canonical CLI schema, including --audit-log-strict and the neighboring audit-log options
• generate authoritative MCP help, validation-error usage, and Bash/Zsh/Fish/PowerShell completions from the shared option inventory
• add parser/help/completion parity coverage and document the discoverability contract

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-build (net8.0: 11,645 passed, 7 skipped; net9.0: 11,166 passed, 423 skipped)
• targeted MCP schema/help/completion tests on net8.0 and net9.0
• dotnet run --project tools/CodeIndex.Changelog -- check
• Codex adversarial review against origin/main (no blocking/actionable findings)

Documentation and changelog

• updated the English and Japanese MCP audit-log sections in USER_GUIDE.md
• added bilingual fragment changelog.d/unreleased/5096.fixed.md

Follow-up candidates

None.

Fixes #5096