Add Bootstrap section to AGENTS.md (#25586)

* Add Bootstrap section to AGENTS.md

Agents reading `AGENTS.md` had no canonical entry point for getting the repo to a buildable state.
The information lived in `README.md`'s "Getting Started" section but wasn't surfaced where agents look.

Surfacing `bundle exec rake dependencies` as the bootstrap command is especially useful in a worktree workflow, where `WordPress/Frameworks/` is gitignored and must be re-populated per worktree.

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Generated with the help of Claude Code, https://code.claude.com

Co-Authored-By: Claude Code Opus 4.7 <noreply@anthropic.com>

* Remove AI verboseness

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Gio Lodi <giovanni.lodi42@gmail.com>

* Add missing new line

---------

Co-authored-by: Claude Code Opus 4.7 <noreply@anthropic.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: 3 people

AGENTS.md

@@ -8,6 +8,16 @@ WordPress for iOS is the official mobile app for WordPress that lets users creat
 
 Minimum requires iOS version is iOS 17. The latest iOS version is iOS 26.
 
+## Bootstrap
+
+To prepare a fresh clone or worktree to build the app, run:
+
+```sh
+rake dependencies
+```
+
+This is the canonical entry point for getting the repo ready to build.
+
 ## High-Level Architecture
 
 ### Project Structure