docs: add Huntley testimonial card and agentic loop diagram to README

[amrynsky]

What changed

Two above-the-fold graphics added to the main README to strengthen the landing page and improve star conversion:

• Geoffrey Huntley testimonial card — the existing quote is now a two-column card with his avatar beside it (name + "creator of Ralph Wiggum" underneath). Puts a face on the social proof.
• Agentic loop diagram — a compact REASON → ACT → OBSERVE mermaid flowchart (handDrawn/neutral, matching repo convention) placed right after the quote, so it visually answers his "show me the inferencing loop" line. Caption links into 01-foundations.

Why

The repo is getting real traffic right now. These are the two highest-leverage above-the-fold conversion levers: social proof (a face on the quote) and a concept diagram of the core loop the tutorials teach.

New asset

common/images/ghuntley.png — 460×460 circular avatar with an anti-aliased alpha mask. Committed pre-rounded because GitHub strips CSS border-radius from README HTML, so the circle has to be baked into the file. (~280 KB; passes the pre-commit large-file check.)

Reviewer notes

• The HTML table and the mermaid block only render on GitHub itself — local markdown preview won't show them. View the rendered README to check.
• Avatar is referenced as a local file (not a hotlink to github.com/ghuntley.png) so the README is self-contained and the rounded image renders.
• The quote was already in the README, publicly sourced (the linked YouTube clip) and attributed; this just adds his public avatar beside it.

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