README improvements - screenshots, demo GIF, walkthrough

[ai-mindset]

Problem

The README has no visuals. Prospective users and clients have no way to
understand what the output looks like without running the tool themselves.

Proposed additions

• Screenshot of Obsidian graph view showing a linked wiki
• Screenshot of a rendered zettel with ## Related links
• Screenshot of mix wiki.search output in the terminal
• GIF or short walkthrough showing: add a URL → link → open in Obsidian

Placement

See discussion in PR for agreed structure.