docs: document PAGES.md template format for agentic-wiki-writer

[Copilot]

docs/agentic-wiki-writer.md had no documentation on the PAGES.md file that drives wiki structure — how it's generated, what the format is, or how to customize it.

Changes

• New PAGES.md Format section in docs/agentic-wiki-writer.md covering:
  • Lifecycle: auto-generated on first run as a PR, freely editable afterward, or supply your own before the first run
  • Header hierarchy (H1–H3 → separate pages; H4+ → in-page sections; ####+ → sidebar-included sections)
  • *{ query }* instruction blocks for AI-generated content alongside preserved static text
  • Header normalization (H4→H2, H5→H3, H6→H4 in rendered pages)
  • Slug generation rules (spaces→hyphens, special chars stripped)
  • Complete annotated example showing input PAGES.md → output files + generated _Sidebar.md

Example of the ####+ sidebar section syntax now documented:

# API Reference

*{ Overview of the API }*

####+ Authentication
*{ Describe auth flow }*

#### Internal Details
*{ Implementation details - not shown in sidebar }*

Content verified against the actual workflow implementation in workflows/agentic-wiki-writer.md.

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