Harden dependency, CI, release, and agent automation

[harder]

Summary

• Add NuGet lock files, SDK/Actions Dependabot coverage, pinned Actions, three-OS CI, four-RID AOT smoke, dependency review, and C#/Actions CodeQL.
• Add a tested daily monitor that opens assigned compatibility issues for new Terminal.Gui and GitHub CLI releases; make both minimum and latest gh contract lanes required.
• Stage releases as verified drafts, refuse published tag reruns, verify published assets for package-manager generation, and update release guidance.
• Refresh AGENTS.md as project source of truth and replace stale Copilot instructions with bot-specific review/coding guidance that references it.

Validation

• actionlint 1.7.12
• node --test .github/scripts/critical-dependencies.test.js
• python site/build.py and node --check site/app.js
• dotnet restore --locked-mode, dotnet build -c Release --no-restore, dotnet test -c Release --no-build (828 passed)
• Sequential locked Native AOT publishes of both hosts for win-arm64; --version smoke passed

Repository settings

The current PAT returns HTTP 403 for repository administration, so Dependabot security updates, release immutability, and a stricter main ruleset still require a repository administrator to enable them in Settings. Workflow code does not bypass that restriction.

[github-advanced-security]

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For more information about GitHub Code Scanning, check out the documentation.