feat(cli): replace open vscode container with devcontainer subagent (#18765)

This change allows a devcontainer to be opened via the agent syntax,
`coder open vscode <workspace>.<agent>` and removes the `--container`
option to simplify the subcommand. Accessing the subagent will behave
similarly to how the `--container` option behaved.

Fixes coder/internal#748

cli/ping.go

@@ -110,7 +110,7 @@ func (r *RootCmd) ping() *serpent.Command {
 			defer notifyCancel()
 
 			workspaceName := inv.Args[0]
-			_, workspaceAgent, err := getWorkspaceAndAgent(
+			_, workspaceAgent, _, err := getWorkspaceAndAgent(
 				ctx, inv, client,
 				false, // Do not autostart for a ping.
 				workspaceName,