[codex] Add suggestion issue draft export

[Widthdom]

Summary

• Add cdidx suggestions export --format issue-drafts with issue-ready title, labels, evidence paths, body text, and optional --open-issues duplicate preflight.
• Extend suggest_improvement to record normalized repository-relative evidencePaths and include them in local exports and GitHub suggestion bodies.
• Use existing repository labels for GitHub suggestion issues: enhancement for ordinary suggestions and bug for crash/error reports.

Changelog

• changelog.d/unreleased/2878.added.md
• changelog.d/unreleased/2931.fixed.md

Validation

• dotnet format CodeIndex.sln --verify-no-changes
• dotnet build
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~ProgramCliTests|FullyQualifiedName~GitHubIssueReporterTests|FullyQualifiedName~McpServerTests.SuggestImprovement"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Adversarial review: No blocking/actionable issues found.

Fixes #2878
Fixes #2931