Add Copilot instructions, prompts, MCP server config, and auto-maintenance workflow

[Copilot]

• Explore repository structure, conventions, and modules
• Create .github/copilot-instructions.md
• Create .github/prompts/add-extension-method.prompt.md
• Create .github/prompts/add-specification.prompt.md
• Create .github/prompts/fix-code-analysis.prompt.md
• Create .github/prompts/create-module.prompt.md
• Create .github/prompts/maintain-copilot-instructions.prompt.md
• Create .vscode/mcp.json
• Create .github/workflows/maintain-copilot-instructions.yml
• Fix pre-existing build failure: CA2251 in SharedCode.Core/Text/StringExtensions.cs — replace string.CompareOrdinal(a, b) == 0 with string.Equals(a, b, StringComparison.Ordinal)
• Fix pre-existing build failure: CS1061 in SharedCode.Data.EntityFramework — add Microsoft.EntityFrameworkCore.Relational package reference so AsSplitQuery() resolves correctly

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[wforney]

@copilot The build is failing...

[Copilot]

@copilot The build is failing...

Fixed in 6601bf2. The failure was a pre-existing CA2251 in SharedCode.Core/Text/StringExtensions.cs — string.CompareOrdinal(a, b) == 0 was replaced with string.Equals(a, b, StringComparison.Ordinal) as the analyzer requires.

[wforney]

@copilot The build failed again.

[Copilot]

@copilot The build failed again.

Fixed in e05a803. The CA2251 fix unmasked a second pre-existing error: CS1061 — AsSplitQuery() is from Microsoft.EntityFrameworkCore.Relational which wasn't referenced by the EF project. Added the package to Directory.Packages.props and Data.EntityFramework.csproj. Full solution now builds with 0 errors.