docs(cli): document the completion command

opencode completion appears in the CLI help but is missing from the CLI
reference. Also note that the credentials command is available as
opencode providers.

Author: oke3

packages/web/src/content/docs/cli.mdx

@@ -129,7 +129,7 @@ opencode attach http://10.20.30.40:4096
 
 ### auth
 
-Command to manage credentials and login for providers.
+Command to manage credentials and login for providers. Also available as `opencode providers`.
 
 ```bash
 opencode auth [command]
@@ -655,6 +655,24 @@ opencode upgrade v0.1.48
 
 ---
 
+### completion
+
+Generates a shell completion script for the CLI.
+
+```bash
+opencode completion
+```
+
+The script is written to stdout, so append it to your shell profile to enable tab completion for `opencode`.
+
+```bash
+opencode completion >> ~/.bashrc
+```
+
+On macOS, append it to `~/.bash_profile` instead.
+
+---
+
 ## Global Flags
 
 The opencode CLI takes the following global flags.