Add mutation testing and enforce CI quality gates

[cangelosilima]

Mutation testing now participates in the required aggregate Quality gate on pull requests and merge-queue commits. Core must reach 90%, the investigated CLI command/progress files 55%, and site reachability 100%. Broader weekly/manual runs remain report-only until those scopes have measured baselines.

Adds pinned Stryker.NET/StrykerJS tooling, local runners, uploaded HTML/JSON reports, and PR check results. Regression tests cover credentials, configuration, discovery, serialization, scheduler cancellation/fairness, CLI options, and reachability. Small production refactors allow Stryker to instrument methods previously excluded after compiler recovery.

Validation:

• Isolated branch: .NET suites passed with 1,026 tests passing and 21 skipped; pre-push checks confirm 100% CLI line, branch, method, and full-method coverage.
• Site CI mutation configuration: 100%, 39 killed and 4 timed out, no survivors.
• Earlier investigation: full Core 91.60%; targeted CLI 56.32%; serialization 97.83%. See docs/mutation-investigation.md for scopes and remaining exclusions.
• Actionlint passed for all workflows; changed site files passed Prettier. Runner checks verified multiple patterns, threshold forwarding, and failure propagation.
• Locked npm dependency installation reported no vulnerabilities.
• Site formatting, lint, and TypeScript checks passed; 540 site tests passed with one skipped and 100% statement, branch, function, and line coverage.

Branch protection must require Quality gate to enforce the aggregate check before merging. This PR does not change repository protection settings.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-26T17:38:46.975604Z	903b637	PR opened

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review" or "@codex security review".

Codex reacts with 👀 while any review is running, comments if it has suggestions, and reacts with 👍 once all reviews finish with no findings.

[github-actions]

Quality results

Quality gate: success
Commit: 45bffe6773b9b0760c19b1a15b3976c288c7f5e1 · View run and reports

Check	Result
Dependency vulnerabilities	success
CodeQL	success
CLI lint and format	success
CLI tests and coverage	success
CLI mutation (Core ≥90%, selected command files ≥55%)	success
CLI performance budgets	success
Site lint and format	success
Site tests and coverage	success
Site reachability mutation (100%)	success
Workflow lint	success
Coverage badge data	success

Line coverage	Measured	Minimum
CLI	100%	100%
Site	100%	80%

CLI line, branch, method, and full method coverage must each reach 100%. A missing report is shown as unavailable. See the linked run for test failures, detailed CLI/Site coverage summaries and HTML reports, and scanner results.