docs: warn that wiki must be initialized before running agentic-wiki-writer (#265)

* Initial plan

* docs: add warning that wiki must be initialized before running agentic-wiki-writer

Co-authored-by: mrjf <180956+mrjf@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: mrjf <180956+mrjf@users.noreply.github.com>

Author: Copilot

docs/agentic-wiki-writer.md

@@ -6,6 +6,9 @@
 
 The [Agentic Wiki Writer workflow](../workflows/agentic-wiki-writer.md?plain=1) keeps your project's GitHub wiki synchronized with the codebase. After each merged pull request (or on demand), it reads a `PAGES.md` template to understand what to document, then writes wiki pages directly from the source code.
 
+> [!WARNING]
+> **The repository wiki must be initialized before running this workflow.** GitHub does not create the wiki git repository until at least one page exists. Go to your repository's **Wiki** tab and create a blank page (e.g. "Home") to initialize it. The workflow will fail with a git clone error if this step is skipped.
+
 ## Installation
 
 ```bash