Give Omegacode a plugin-owned Omega icon

[brsbl]

Summary

• make Omegacode own its compact Omega artwork at assets/icon.svg
• point the existing bb.branding.icon field at that plugin-relative SVG
• validate that plugin-owned compact assets are included in production packages and generated install refs

The stable package name (bb-plugin-omega), plugin id (omega), screenshots, and runtime behavior are unchanged. Older bb 0.0.32 hosts continue to load the plugin and show their generic fallback. A host with the generic compact-icon contract renders the local SVG as a monochrome currentColor mask.

The corresponding generic bb host commit is fe46ef1c3 on origin/bb/plugin-owned-compact-icon; it has been handed to thr_4z84idik9a for the combined bb release. This PR does not require an Omegacode-specific host glyph.

Validation

• npm run check --workspace=bb-plugin-omega
• npm run check
• npm run hygiene
• focused artifact validation (omega, 13 packed files)
• 5 Omegacode test files / 24 tests
• scaffold smoke and all five plugin artifact checks
• git diff --check

A staged review loop found no P0-P2 issues. This PR must not be merged without explicit user approval.