docs: add key repo files section

docs/ai/overview.md

@@ -48,6 +48,46 @@ Copilot is an AI coding assistant that integrates with editors and chats to help
 - Skills: Reusable knowledge or workflows the assistant can apply.
 - Tokens: The usage units that track AI consumption.
 
+## Key Repo Files (Why They Exist)
+These files explain how the AI docs are organized and how contributors should work. Each one has a different job.
+
+### Agents.md
+What it is: The workflow rules for using AI in this repo.
+
+Why you create it: It keeps requests consistent so changes are scoped, reviewable, and repeatable.
+
+What to include:
+- Request structure (goal, inputs, output, verification).
+- Plan-first guidance for multi-step changes.
+- Sync rule: keep PRD.md and README.md aligned when workflows or scope change.
+
+Project example: See [Agents.md](../../Agents.md) for the request template and contribution workflow used in this repo.
+
+### PRD.md
+What it is: The product requirements for the documentation set.
+
+Why you create it: It anchors scope and success metrics so the docs do not drift.
+
+What to include:
+- Problem statement, goals, and non-goals.
+- Target audience and user stories.
+- Content requirements (step-by-step guidance and examples).
+- Definition of done and sync expectations.
+
+Project example: See [PRD.md](../../PRD.md) for the requirements and definition of done used here.
+
+### README.md
+What it is: The entry point for contributors and readers who need quick orientation.
+
+Why you create it: It tells people what the repo is, where to start, and how to contribute.
+
+What to include:
+- Repo purpose and audience.
+- Where to start (index, overview, setup guides).
+- Contribution habits and publishing steps.
+
+Project example: See [README.md](../../README.md) for the start-here links and local workflow.
+
 ## Chat Types In Plain English
 Copilot chat has four modes. Pick the smallest one that fits the task.