[codex] Improve MCP self-guiding metadata

[Widthdom]

Summary

• Strengthen MCP initialize instructions with CodeIndex-first investigation guidance and focused excerpt flow.
• Clarify major tools/list descriptions and common schema descriptions so clients know when to use each tool and what to do next.
• Add self-guiding prompts for investigate-before-edit, existing pattern discovery, safe symbol changes, and failure debugging.
• Add next-step, recovery, and pagination hints to read-only MCP responses.
• Add a bilingual unreleased changelog fragment.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~Mcp" -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json (index_matches_workspace: true)