chore: enable agent socket by default (#22352)

relates to #21335

Enables the agent socket by default and updates docs to strike references to having to enable it.

The PRs in this stack change the MCP server that Tasks use to update their status to rely on the agent socket, rather than directly dialing Coderd with the agent token.

Default disable was a reasonable default when it was only used for the experimental script ordering features, but now that we want to use it for Tasks, it should be default on.

testutil/unixsocket.go

@@ -22,7 +22,7 @@ import (
 //
 // On Linux, we also hit this limit on GitHub Actions runners where TMPDIR is
 // set to a long path like /home/runner/work/_temp/go-tmp/.
-func TempDirUnixSocket(t *testing.T) string {
+func TempDirUnixSocket(t testing.TB) string {
 	t.Helper()
 	// Windows doesn't have the same unix socket path length limits,
 	// and callers of this function are generally gated to !windows.
@@ -41,7 +41,7 @@ func TempDirUnixSocket(t *testing.T) string {
 	return dir
 }
 
-func AgentSocketPath(t *testing.T) string {
+func AgentSocketPath(t testing.TB) string {
 	if runtime.GOOS == "windows" {
 		return fmt.Sprintf(`\\.\pipe\com.coder.agentsocket_test.%s.%s`, t.Name(), rand.Text())
 	}