fix: handle agent parameter in URIs (#538)

This fixes a bug where the `agent` query parameter on the extension URI was ignored.

We were previously doing:
```ts
vscode.commands.executeCommand(
	"coder.openDevContainer",
	workspaceOwner,
	workspaceName,
	workspaceAgent,
	devContainerName,
	devContainerFolder,
);
```
where `args[2]` was the agent name, but we were discarding it:
```ts
const workspaceOwner = args[0] as string;
const workspaceName = args[1] as string;
const workspaceAgent = undefined; // args[2] is reserved, but we do not support multiple agents yet.
const devContainerName = args[3] as string;
const devContainerFolder = args[4] as string;
```

The same was true for the `coder.open` command. Presumably due to the comment saying multiple agents aren't supported, which hasn't been true for years.

Author: ethanndickson

.git-blame-ignore-revs

@@ -0,0 +1,5 @@
+# If you would like `git blame` to ignore commits from this file, run:
+# git config blame.ignoreRevsFile .git-blame-ignore-revs
+
+# chore: simplify prettier config (#528)
+f785902f3ad20d54344cc1107285c2a66299c7f6

src/commands.ts

@@ -600,7 +600,7 @@ export class Commands {
 		} else {
 			workspaceOwner = args[0] as string;
 			workspaceName = args[1] as string;
-			// workspaceAgent is reserved for args[2], but multiple agents aren't supported yet.
+			workspaceAgent = args[2] as string | undefined;
 			folderPath = args[3] as string | undefined;
 			openRecent = args[4] as boolean | undefined;
 		}
@@ -628,7 +628,7 @@ export class Commands {
 
 		const workspaceOwner = args[0] as string;
 		const workspaceName = args[1] as string;
-		const workspaceAgent = undefined; // args[2] is reserved, but we do not support multiple agents yet.
+		const workspaceAgent = args[2] as string;
 		const devContainerName = args[3] as string;
 		const devContainerFolder = args[4] as string;
 
@@ -748,7 +748,7 @@ async function openDevContainer(
 	baseUrl: string,
 	workspaceOwner: string,
 	workspaceName: string,
-	workspaceAgent: string | undefined,
+	workspaceAgent: string,
 	devContainerName: string,
 	devContainerFolder: string,
 ) {