[codex] Report source detector rejection reasons

[Widthdom]

Summary

• Added SourceCodeDetector.Detect(...) with stable source-code guard reason codes while keeping ContainsSourceCode(...) as the compatibility boolean wrapper.
• Added tilde Markdown fence and indented fence coverage, plus bounded MCP source_code_rejection diagnostics that do not echo rejected text.
• Updated user/developer docs and added the bilingual changelog fragment.

Validation

• dotnet format CodeIndex.sln --verify-no-changes
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Debug -p:UseSharedCompilation=false --filter "FullyQualifiedName~SourceCodeDetectorTests|FullyQualifiedName~McpServerTests.SuggestImprovement_SourceCode"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln -c Debug -p:UseSharedCompilation=false
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• codex exec review --dangerously-bypass-approvals-and-sandbox --base origin/main

Review

• Adversarial review: No blocking/actionable issues found.
• Review feedback about preserving list-indented fenced block detection was addressed in the second Return source-code detector reasons and cover Markdown fence variants #3830 commit. git commit --amend was blocked by the local environment guard, so the fix remains as a separate same-issue commit.

Scope Notes

• No additional related issues were included.
• No new GitHub issues were created.
• AGENT.md, AGENTS.md, CLAUDE.md, and AGENT_GUIDE.md were not modified.

Fixes #3830