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

[yves-vogl]

Summary

• Adds the Nave brand icon as part of a suite-wide branding rollout across the Metal up your ass plugin family — a gold-on-charcoal heraldic emblem set, one mark per plugin.
• Embeds the icon centered above the title in README.md (160px) and at the top of docs/manual.md (120px).

Details

• docs/assets/icon.png — 1024x1024 source icon (generated brand asset).
• docs/assets/icon-256.png — 256px resized variant (via sips -Z 256).
• Both images are licensed under this repo's AGPLv3 license, same as the rest of the codebase.
• Docs-only change — no source, build, or CI configuration touched.

Test plan

• Verified image paths resolve correctly from each file's location (docs/assets/icon.png from repo root in README.md; assets/icon.png from docs/manual.md).
• Confirmed docs/assets/icon-256.png is a valid 256x256 PNG via file.
• CI green (docs-only change, no functional impact expected).

🤖 Generated with Claude Code