Replace Codecov with ReportGenerator coverage gate

[craigktreasure]

Summary

Codecov was sold to Harness. This replaces the upload with a fully in-repo approach using the ReportGenerator dotnet tool — no third-party GitHub Actions involved:

• Coverage summary rendered into the workflow job summary (MarkdownSummaryGithub report type)
• HTML report uploaded as a build artifact for line-by-line drill-down, named per matrix leg
• Coverage gate: minimumCoverageThresholds fails the build when line or branch coverage falls below the floor
• Tool is version-pinned in .config/dotnet-tools.json, restored by the existing install-tools action, and already covered by Dependabot's nuget updates
• The dotnet-coverage global install and merge steps are gone — ReportGenerator reads all nine cobertura files directly
• Removes codecov.yml and the README badge
• Drops secrets: inherit from CI.yml and PR.yml, since CODECOV_TOKEN was the only secret the build workflow used

Thresholds

Both line and branch coverage are gated at 95%, matching the target the old codecov.yml used. Current coverage is 100% line and 98.8% branch.

Source-generated regex code is excluded via -classfilters. It appears three times (once per TFM) and accounts for the entire gap between the raw 89.9% line coverage and the 100% of hand-written code. Codecov never counted it either — those files live under __artifacts/obj and do not exist in the commit, which is what made a 95% target viable there.

Notes

• Multiple thresholds must be passed as separate arguments. Combining them as lineCoverage=95;branchCoverage=95 throws a config-binder exception and still exits 0.
• On a breach, ReportGenerator writes the reports before exiting non-zero, so the !cancelled() guards on the summary and artifact steps still publish.
• The summary step uses pwsh rather than cat >> "$GITHUB_STEP_SUMMARY" so it works on the windows-2025 leg of the PR matrix.
• .config/** is in CI.yml's paths-ignore, so Dependabot bumps of the tool will not trigger a CI build on main. They still run the full PR matrix, so the gate is exercised.