Add README.md to every directory

Each subdirectory now has a README with:
- File inventory table
- Usage guidelines and specifications
- Technical details (viewbox, colors, sizes)
- Cross-references to related assets

icons/README.md

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+# Icons
+
+Small-format brand marks for digital use.
+
+## Subdirectories
+
+| Directory       | Contents                              |
+|-----------------|---------------------------------------|
+| `favicon/`      | Browser favicon and touch icons       |
+| `social-media/` | Profile images for social platforms   |

icons/favicon/README.md

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+# Favicon
+
+Browser favicon for keylinkit.com and related web properties.
+
+## Files
+
+| File                   | Description                                       |
+|------------------------|---------------------------------------------------|
+| `keylink-favicon.png`  | Circular K colormark, white on cobalt blue        |
+
+## Implementation
+
+Use at 16x16, 32x32, and 180x180 (Apple touch icon). The circular crop of the K monomark ensures readability at small sizes.
+
+```html
+<link rel="icon" type="image/png" href="/favicon.png">
+<link rel="apple-touch-icon" sizes="180x180" href="/apple-touch-icon.png">
+```

icons/social-media/README.md

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+# Social Media Icons
+
+Profile and avatar images sized for social media platforms. Use the K monomark on blue background for square crops, or the circular favicon/colormark for circular crops.
+
+Currently empty. Generate from `../favicon/keylink-favicon.png` or `../../logos/monomark/` assets at these recommended sizes:
+
+| Platform    | Size      |
+|-------------|-----------|
+| Facebook    | 170x170   |
+| LinkedIn    | 400x400   |
+| X / Twitter | 400x400   |
+| Instagram   | 320x320   |
+| Google      | 250x250   |