Add ad-hoc codesigning to build script and fix install instructions

Prevents the 'damaged and can't be opened' Gatekeeper error for
downloaded builds. README now includes the xattr workaround.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Rename to OneCC and add dog chibi app icon

Generate pixel-art dog icon from ChibiView at all macOS sizes with dark
rounded-rect background and subtle glow. Update package name, bundle ID,
build script, Info.plist, README, and CLAUDE.md from OCC to OneCC.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Add buddies showcase to README with ASCII art for all 17 chibis

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>