Add CI coverage, formatting policy, and dev task wrappers

[Widthdom]

Summary

• Collect XPlat Code Coverage in CI and upload per-lane Cobertura artifacts.
• Add .editorconfig, enforce dotnet format, and treat compiler warnings as errors with an explicit trim-warning allowlist.
• Add shared developer task wrappers via Makefile and dev.sh, document them, and verify the wrapper path in CI.

Validation

• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore
• make lint
• make build
• make mcp-smoke
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net8.0 --filter "FullyQualifiedName~RunPublishedTrimmedCli_SerializesQueryJsonAndErrorJson" --settings tests/CodeIndex.Tests/CodeIndex.Tests.runsettings --blame-crash --blame-hang --blame-hang-timeout 5m
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

Note: a full make test run initially failed only on RunPublishedTrimmedCli_SerializesQueryJsonAndErrorJson because warning-as-error made existing trim warnings fatal; after adding the explicit allowlist and ILLinkTreatWarningsAsErrors=false, the failing test passed.

Documentation and Changelog

• Updated CONTRIBUTING.md and DEVELOPER_GUIDE.md.
• Added changelog fragments:
  • changelog.d/unreleased/1608.fixed.md
  • changelog.d/unreleased/1609.internal.md
  • changelog.d/unreleased/1611.internal.md

Review

• Adversarial review: No blocking/actionable issues found.
• External codex exec review attempt was blocked by usage limits, so the same workflow was applied manually in this Codex session.

Fixes #1608
Fixes #1609
Fixes #1611