docs: desktop install + OpenCode command linking (#105)

Documents the desktop-app install flow (no /plugin command) and the global command-dir linking needed for /ponytail commands in OpenCode outside a checkout. Covers the recurring questions in #97 and #98.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: DietrichGebert

README.md

@@ -101,6 +101,8 @@ The Claude Code and Codex plugins run two tiny Node.js lifecycle hooks, so `node
 /plugin install ponytail@ponytail
 ```
 
+The desktop app has no `/plugin` command. Install it from the UI instead: Customize, the + by personal plugins, Create plugin and add marketplace, Add from repository, then enter the repo URL (thanks @NiklasDHahn, #98).
+
 ### Codex
 
 ```bash
@@ -152,6 +154,8 @@ Injects the ruleset every turn at the active level; adds the `/ponytail` command
 
 The `./` path resolves against your project's `opencode.json`; to share one checkout across projects, point it at the absolute path of the `.mjs` instead (it finds its `hooks/` and `skills/` relative to its own file).
 
+The plugin path loads the ruleset everywhere, but the `/ponytail` commands are separate files in `.opencode/command/` that OpenCode only discovers from your project or the global commands dir. To use them outside this checkout, link them once: `ln -sf /absolute/path/to/ponytail/.opencode/command/* ~/.config/opencode/command/`.
+
 ### Gemini CLI
 
 ```bash