fix(#194): slim agentic Copilot workflow and add wire-up integrity test

 - Move cross-cutting rules into .github/instructions/repository-conventions.instructions.md and slim copilot-instructions.md to a thin index with a task → prompt → agent →
skills → instructions table
 - Generate AGENTS.md from copilot-instructions.md via Sync-AgenticCopilotScaffold.ps1 with a do-not-edit banner; remove AGENTS.md from scaffold-owned paths
 - Make scaffold TextReplacements code-fence-aware (skip fenced blocks and inline backticks) via new Split-AgenticContentByCodeFence and Invoke-AgenticInlineCodeAwareReplace
helpers
 - Rename .github/skills/docs-site-html → docs-site to match the agent name and update references
 - Delete .github/prompts/markdown.prompt.md and fold its wrapper rules and example into the markdown-authoring skill
 - Normalize prompt headers and add an explicit "Invoke with @.github/prompts/<name>.prompt.md" line that names the delegated agent
 - Add "When unclear, ask before proceeding" clause to reviewer and release-manager agents
 - Replace the anchorless one-liner in code-quality-matrix.instructions.md with the ten concrete maintainability dimensions
 - Add tests/AgenticCopilotWiring.Tests.ps1 asserting /skill references, prompt → agent references, instruction applyTo: globs, and skill name/folder parity
 - Update scaffold-sync tests for the slim structure (new RepositoryConventions field, AGENTS.md banner assertion) and drop markdown.prompt.md from CoverageGaps expectations

Author: stiwicourage