Add repository-root AGENTS.md with agent-specific operating guidance

[Copilot]

The issue requested adding a new AGENTS.md file at the repository root (only if missing) with actionable agent guidance tailored to this repository’s structure and workflows. This PR adds that file with project-specific instructions aligned to the repo’s documentation-first, module-based layout.

• Project context for agents

  • Documents repository purpose, module-oriented structure, and where key governance files live (CONTRIBUTING.md, SECURITY.md, workflows).

• Execution and workflow guidance

  • Adds concrete setup/navigation/search commands appropriate for this repo (no single root package project).
  • Defines how to scope edits to lesson modules and preserve existing file/link conventions.

• Validation and contribution expectations

  • Captures practical pre-PR checks for this repo type (diff/path/reference verification).
  • Includes PR-focused guidance and security handling expectations consistent with repository policies.

## Testing instructions

There is currently no repository-wide automated lint/test/build command at the root.

Before submitting:
- Check Markdown and links manually in changed files.
- Validate changed file paths and references:
  - `git diff --name-only`
  - `git diff --check`