docs(branding): add plugin icon and embed in README and manual

[yves-vogl]

Summary

• Adds the Triptych brand icon as part of a suite-wide branding rollout across the Metal up your ass plugin family.
• The icon is a generated brand asset: a gold-on-charcoal heraldic emblem, consistent with the visual identity being applied across the suite.
• Embeds the icon at the top of README.md (160px, centered) and docs/manual.md (120px, centered) so it shows up both on the GitHub repo page and in the user manual.
• Adds a 256px downscaled variant (docs/assets/icon-256.png) alongside the full-resolution source (docs/assets/icon.png) for future use (e.g. app icon generation, favicons).
• This asset is committed to this AGPLv3-licensed repository and is distributed under the same license as the rest of the codebase.

Test plan

• Confirm docs/assets/icon.png and docs/assets/icon-256.png render correctly on GitHub
• Confirm the icon displays correctly at the top of the rendered README
• Confirm the icon displays correctly at the top of docs/manual.md
• No CI impact expected (docs/assets-only change)