Explain ambiguous language candidates

[Widthdom]

Summary

• Explain ambiguous .m and .pl extension candidates from detector-owned descriptors in CLI and MCP languages output.
• Report the detector's filename, shebang, content, and project precedence with bounded evidence and override guidance.
• Report shared source/confidence metadata for actual dry-run selections while preserving the bounded full-scan decision.

Validation

• dotnet format CodeIndex.sln --no-restore --verify-no-changes
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore
• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-build --no-restore (net8.0: 10,983 passed; net9.0: 10,509 passed; 0 failed after rerunning the unrelated flaky net8.0 test/run)
• Focused Issue Explain candidate languages for ambiguous file extensions #4901 tests on net8.0 and net9.0
• MCP output-schema compatibility tests
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

Documentation and changelog

• Updated the English and Japanese sections of USER_GUIDE.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md.
• Added changelog.d/unreleased/4901.added.md.

Review

• Codex adversarial review: No blocking/actionable issues found.

Fixes #4901