Consolidate instruction files into skills — single source of truth

Delete 7 instruction files whose content is now fully covered by skills:
- local-sdk-workflow.instructions.md → azsdk-common-generate-sdk-locally
- create-release-plan.instructions.md → azsdk-common-prepare-release-plan
- sdk-details-in-release-plan.instructions.md → azsdk-common-prepare-release-plan
- verify-namespace-approval.instructions.md → azsdk-common-prepare-release-plan
- check-package-readiness.instructions.md → azsdk-common-sdk-release
- check-package-validation.instructions.md → azsdk-common-generate-sdk-locally
- copilot/sdk-release.instructions.md → azsdk-common-sdk-release

Skills updated with all unique details from instruction files:
- generate-sdk-locally: added commit checkpoints, language selection,
  config file identification, SDK project path, metadata update tools
- prepare-release-plan: added critical LLM instructions (NextSteps field),
  Release Planner Tool link, package name extraction rules per language,
  valid/invalid examples table, JSON structure, namespace approval flow
- sdk-release: added readiness check details, interaction flow pattern,
  do-not-ask-for-PR/plan guard, case-sensitive language note

typespec-to-sdk orchestrator updated to reference skills directly.
Removed all related_skill frontmatter and blockquotes.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 2 people