feat: add OpenAI Shadcn Desktop theme

[agentik-os]

Summary

Adds openai-shadcn-desktop, a directly installable Hermes Desktop theme/plugin:

• accessible neutral/zinc light and dark palettes through THEMES_AREA
• floating opaque pane shell with responsive gutters and 14–16 px radii
• refined composer alignment, separator-level borders, and restrained shadows
• subtle dot-grid shell background
• consistent professional hover states
• command-palette activation and one-click rollback to the Hermes default
• safe public preview and standalone distribution link

Closes #13.

Why this repository

The Desktop Plugin SDK points authors to this companion repository, but there is not yet a directly installable Desktop theme example. This demonstrates a production-shaped theme contribution and how layout polish can remain strictly gated by the selected theme.

Compatibility

The plugin does not replace the layout tree or routing. Resize, collapse, drag, sessions, profiles, registered gateways, Local/VPS, Files, Terminal, and Review remain core-owned. Selecting another theme removes every custom layout rule immediately.

Verification

• node --check openai-shadcn-desktop/plugin.js
• git diff --check
• loaded in Hermes Desktop v0.20.4 on macOS
• verified dark and light palettes in an isolated renderer
• verified the packaged app with existing sessions, Files pane, registered Local/VPS gateways, and Fleet Sessions
• no core files modified

Guideline note

This is intentionally larger than a minimal single-method reference because a complete dual-mode palette is data-heavy and the floating layout is the user-facing example. The plugin still uses one coherent surface: a contributed Desktop appearance. If maintainers prefer a theme-only subset, I can split the palette reference from the full standalone plugin.