docs(ai-agents): design doc for context-aware next-step guidance

Initial draft of the design for issue #7975  context-aware Next: guidance
across azd ai agent commands plus a new `azd ai agent doctor` diagnostic.

Scoped entirely to cli/azd/extensions/azure.ai.agents/. References core
files for context but proposes no edits outside the extension.

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

Author: Antriksh Jain