Add USAGE.md: per-plugin and per-MCP usage guide

Documents what each of the 8 plugins and 14 MCP servers does, how to use it
(auto vs slash commands/tools), and prerequisites. Links it from the README
and adds enable/disable instructions.

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

README.md

@@ -72,6 +72,11 @@ echo 'export BRAVE_API_KEY=BSA...' >> ~/.bashrc
 - **14 MCP Servers** - codegraph, brave-search, playwright, github, context7, exa, etc.
 - **Model Config** - Kimi K2.6 + 14 other models via `@ai-sdk/openai-compatible`
 
+## Usage
+
+See **[USAGE.md](USAGE.md)** for what every plugin and MCP server does, how to use it,
+and its prerequisites — plus how to enable/disable each one.
+
 ## Updating
 
 ```bash
@@ -114,6 +119,7 @@ git pull
 ```
 opencode-config/
 ├── opencode.json.template    # Config template with placeholders
+├── USAGE.md                  # Per-plugin & per-MCP usage guide
 ├── agents/                   # Agent definitions
 │   ├── explorer.md
 │   ├── reviewer.md
@@ -124,5 +130,6 @@ opencode-config/
 │   └── ...
 ├── install.ps1              # Windows installer
 ├── install.sh               # macOS/Linux installer
-└── README.md
+├── README.md
+└── USAGE.md
 ```