Document CLI shell completion (viam completion)

rdk#5947 enabled urfave/cli v3 shell completion, adding a hidden
`viam completion` subcommand that outputs completion scripts for
bash, zsh, fish, and PowerShell. Add setup instructions to the CLI
overview and a reference entry for the completion command.

https://claude.ai/code/session_01EFZQ9UyvXPCRPfr1XcAXC7

Author: claude

docs/cli/overview.md

@@ -202,3 +202,51 @@ viam --help
 viam machines --help
 viam machines part shell --help
 ```
+
+## Enable shell completion
+
+The CLI supports tab completion for commands, subcommands, and flag names.
+To enable it, source the completion script for your shell.
+
+{{< tabs >}}
+{{% tab name="bash" %}}
+
+Add to your `~/.bashrc`:
+
+```sh {class="command-line" data-prompt="$"}
+source <(viam completion bash)
+```
+
+{{% /tab %}}
+{{% tab name="zsh" %}}
+
+Add to your `~/.zshrc`:
+
+```sh {class="command-line" data-prompt="$"}
+source <(viam completion zsh)
+```
+
+{{% /tab %}}
+{{% tab name="fish" %}}
+
+```sh {class="command-line" data-prompt="$"}
+viam completion fish > ~/.config/fish/completions/viam.fish
+```
+
+{{% /tab %}}
+{{% tab name="PowerShell" %}}
+
+```powershell {class="command-line" data-prompt=">"}
+viam completion pwsh | Out-String | Invoke-Expression
+```
+
+{{% /tab %}}
+{{< /tabs >}}
+
+After sourcing the script, press **Tab** to complete commands and flags:
+
+```sh {class="command-line" data-prompt="$"}
+viam <Tab>         # lists all commands
+viam machines <Tab> # lists subcommands of machines
+viam data export <Tab> # lists subcommands of export
+```

docs/cli/reference.md

@@ -19,6 +19,28 @@ viam [global options] command [command options] [arguments...]
 
 ## Commands
 
+### `completion`
+
+The `completion` command outputs a shell completion script.
+Source the output in your shell to enable tab completion for commands, subcommands, and flag names.
+
+```sh {class="command-line" data-prompt="$"}
+viam completion bash
+viam completion zsh
+viam completion fish
+viam completion pwsh
+```
+
+<!-- prettier-ignore -->
+| Argument | Description |
+| -------- | ----------- |
+| `bash` | Output a bash completion script. |
+| `zsh` | Output a zsh completion script. |
+| `fish` | Output a fish completion script. |
+| `pwsh` | Output a PowerShell completion script. |
+
+For setup instructions, see [Enable shell completion](/cli/overview/#enable-shell-completion).
+
 ### `data`
 
 The `data` command allows you to manage machine data.