chore: Add Agentic AI development assets for GitHub Copilot

[PlagueHO]

Summary

Add Agentic AI development assets so that GitHub Copilot (agent mode) and other AI coding agents can build, test, and contribute to this repository correctly — following DSC Community conventions and FileContentDsc-specific patterns.

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Files to create

1. AGENTS.md (repository root)

• Repository overview — file content management DSC resources, supported Windows versions
• Resource type — MOF-based resources in source/DSCResources/
• Build commands:

  ./build.ps1 -Tasks noop
  ./build.ps1 -Tasks build
  Invoke-Pester -Path 'tests/Unit' -Output Detailed

• Never run integration tests locally — CI only
• DSC Community resources — Guidelines, Blog Posts

2. tests/AGENTS.md

• Pester 5 syntax required; Pester 4 must not be used
• Should -Invoke, Should -BeTrue, BeforeDiscovery, $PSDefaultParameterValues
• Unit tests: tests/Unit/DSCResources/; Integration tests: tests/Integration/ (CI only)

3. .github/copilot-instructions.md

• FileContentDsc identity, always update CHANGELOG.md, MOF-based resources, instruction file references
• Links: https://dsccommunity.org/guidelines/, https://dsccommunity.org/blog/

4. .github/instructions/*.instructions.md

New file	Adaptations
dsc-community-style-guidelines.instructions.md	DSCResources layout; MOF-only
dsc-community-style-guidelines-mof-resources.instructions.md	Copy as-is
dsc-community-style-guidelines-pester.instructions.md	Copy as-is
dsc-community-style-guidelines-unit-tests.instructions.md	FileContentDsc; tests/Unit/DSCResources/; no SMO
dsc-community-style-guidelines-integration-tests.instructions.md	FileContentDsc; tests/Integration/; no SQL CI notes
dsc-community-style-guidelines-powershell.instructions.md	Copy as-is
dsc-community-style-guidelines-localization.instructions.md	MOF resource string paths
dsc-community-style-guidelines-changelog.instructions.md	dsccommunity/FileContentDsc repo URL
dsc-community-style-guidelines-markdown.instructions.md	Copy as-is
FileContentDsc-guidelines.instructions.md	FileContentDsc-specific: build/test workflow, MOF naming, no SQL/SMO; applyTo: "**"

Acceptance criteria

• AGENTS.md created at repository root
• tests/AGENTS.md created
• .github/copilot-instructions.md created
• All 10 .github/instructions/*.instructions.md files created with no SQL Server–specific content

References

• SqlServerDsc instruction files: https://github.com/dsccommunity/SqlServerDsc/tree/main/.github/instructions