docs: add built-in agents reference to README (#3047)

Co-authored-by: Jay V <[email protected]>

README.md

@@ -51,6 +51,22 @@ OPENCODE_INSTALL_DIR=/usr/local/bin curl -fsSL https://opencode.ai/install | bas
 XDG_BIN_DIR=$HOME/.local/bin curl -fsSL https://opencode.ai/install | bash
 ```
 
+### Agents
+
+OpenCode includes two built-in agents you can switch between,
+you can switch between these using the `Tab` key.
+
+- **build** - Default, full access agent for development work
+- **plan** - Read-only agent for analysis and code exploration
+  - Denies file edits by default
+  - Asks permission before running bash commands
+  - Ideal for exploring unfamiliar codebases or planning changes
+
+Also, included is a **general** subagent for complex searches and multi-step tasks.
+This is used internally and can be invoked using `@general` in messages.
+
+Learn more about [agents](https://opencode.ai/docs/agents).
+
 ### Documentation
 
 For more info on how to configure OpenCode [**head over to our docs**](https://opencode.ai/docs).